Canadian source
Air Brake Manual
• How air brake systems work
•B
rake adjustments and
in-service checks
•P
re-trip inspections
Foreword
The Air Brake Manual has been prepared by Manitoba
Public Insurance to assist drivers in understanding
the basic operation and function of an air brake
system. The study of this manual, together with
practical instruction, is recommended for a driver
who is preparing for the air brake examination. A
large illustration of a complete dual air brake system
is located on the inside cover and can be folded out
and referred to when studying this manual. Study
questions are included at the end of each section so
that readers may self–test their understanding of
the subject matter. Drivers who have qualified
and are authorized to operate air brake-equipped
vehicles are encouraged to review this manual on a
periodic basis to ensure they are fully aware of the
proper method of inspecting an air brake system
and identifying problems that can occur when
the system malfunctions.
The illustrations and explanations of various types of
brake system designs are provided for instructional
purposes only. Most air gauges measure in imperial
units, and so in this manual the measurements used
and relating to the air brake system will be in imperial
units. This manual has no legislative sanction.
For interpreting and applying the law, consult The
Drivers and Vehicles Act, The Highway Traffic Act and
its regulations. Sales of this manual are final and
not refundable.
We gratefully acknowledge the contributions of all
jurisdictions, particularly British Columbia.
Air brake endorsement
This endorsement permits the holder to drive vehicles
equipped with air brakes in the class of vehicle for
which the driver is licenced.
To adjust manual or automatic slack adjusters, the
operator must hold an S brake endorsement.
Requirements for air
brake endorsement
•m
ust complete an air brake written test
• must
complete an air brake practical test
Check the slack!
It is up to you,
the driver, to ensure
that your vehicle
has safe, properlyadjusted brakes.
Air Brake Manual
Foldout 1
An air brake endorsement is not required for a
licensed holder to drive a Class 3 or 5 air brakeequipped vehicle if it is registered as a farm vehicle.
Class 3 or 5 air brake-equipped vehicles, fitted
with dealer plates, may be driven by farmers for
demonstration purposes.
Use of licence as
a learner permit
A holder of any licence may be authorized to operate
vehicles equipped with air brakes as a learner after
meeting the required written standards. The learner
must be supervised by someone who has held, for at
least two years, an air brake endorsement.
29
27
28
26
25
24
23
11
12
22
21
20
19
18
14
15
30
31
6
32
17
33
16
34
1
2
3
Dual air brake system
4
5 6
7
8
6
Foldout 2
10
Supply/wet
Spring parking brake system
Primary/dry
Trailer system
Secondary/dry
Air Brake Manual
9 6
In-cab portion
11
12
13
This illustration is referenced
throughout the manual.
Look for numbers in brackets
within the text (e.g.: The source
of the compressed air is a
compressor (1)).
14
1.Compressor
2.Governor
3.Air dryer
4.Safety valve
5.Supply/wet reservoir
6.Drain valves
7.One–way check valves
8.Primary/dry reservoir
9.Low pressure indicator
10.Secondary/dry reservoir
11.Rear service brake chambers
12.Spring parking brake chambers
13.Tractor relay valves
14.Trailer service brake chamber
15.Trailer spring parking brake chamber
16.Trailer reservoirs
17.Trailer relay valve
18.Trailer spring brake valve
19.Anti–compound lines
20.Glad hands
15
21.Supply (emergency) line
22.Control (service) line
23.Spring brake modulator valve
24.Tractor protection valve
25.Stop lamp switch
26.Two–way check valves
27.Spring parking brake control valve
28.Trailer supply valve
29.Reservoir air pressure gauges
30.Trailer brake hand valve
31.Foot valve
32.Front service brake chambers
33.Quick release valve
34.Automatic front brake limiting valve
This illustration has an automatic front
brake limiting valve (34). The control
valve (35) for a manual front brake
limiting valve (36) are not shown here,
but appear later in the manual.
Table of Contents
Foreword
Air brake endorsement
Requirements for air brake endorsement
Use of licence as a learner permit
Dual air brake system illustration
Foldout 1
Foldout 1
Foldout 1
Foldout 1
Foldout 2
Test appointments
3
Section 1 — Brakes and braking
5
Heat-energy-traction-friction6
Speed-weight-distance7
Braking force
7
Use of air pressure
8
Leverage and air pressure
8
Stopping distance
8
Section summary questions
9
Section 2 — Components of an air brake system
11
Compressor and governor
12
Reservoirs14
Air dryer
15
Safety valve
15
Foot valve
16
Brake chambers, slack adjusters and brake lining
16
Wedge brakes
20
Disc brakes
21
Air-over-hydraulic brake systems
21
Air-actuated-hydraulic brake system
21
Air-boost-hydraulic brake system
23
Section summary questions
24
Section 3 — How the basic air brake system works
One-way check valve
Air pressure gauge Brake application gauge
Low pressure warning device
Stop light switch
Quick release valve
Relay valve
Manual front brake limiting valve
Automatic front brake limiting valve
Tandem rear axles
Section summary questions
25
26
27
27
27
27
27
28
28
29
29
30
Section 4 — Spring parking brake system
Using a spring parking brake
Mechanical release (caging)
Section summary questions
31
33
34
34
Section 5 — Trailer system
Glad hands
Application line
Trailer brake hand valve
Two-way check valves
Tractor protection system
Tractor protection valve
Trailer supply valve
Automatic trailer supply valve system
Tractor and trailer coupled
Charging the trailer system
Foot or hand valve brake application
Emergency application
Supply (emergency) line rupture
Control (service) line rupture
Loss of reservoir air pressure
Manual trailer supply valve
Trailer spring parking brakes
Section summary questions
35
36
36
37
38
39
39
41
42
44
45
45
47
48
48
49
50
50
51
Section 6 — Dual air brake system
Basic dual air brake system
Dual air brake system with spring parking brakes
Spring parking brakes with modulator valve
Combination tractor and trailer with
spring parking brakes
Section summary questions
53
54
55
56
Section 7 — Electronic controlled braking and
traction systems
Anti-lock brake system (ABS)
Automatic traction control (ATC)
Section summary questions
57
58
59
60
62
62
Section 8 — Brake adjustment and in-service check 63
Brake adjustment
64
Brake adjustment — it’s critical
64
Brake adjustment — it’s the law
64
Definitions64
S-cam brake — released
64
S-cam brake — applied
65
S-cam brake — incorrectly adjusted and
cold brake drum
65
S-cam brake — incorrectly adjusted and
hot brake drum
66
Checking and adjusting S-cam brakes — manual slack adjusters
66
Brake adjustment indicators
67
Brake adjustment — manual slack adjuster
68
Adjusting S-cam brakes with automatic slack
adjusters — why, when and how
69
Emergency manual brake adjustment — automatic slack adjusters
70
Disc brake adjustment
72
Wedge brake adjustment
72
After a brake adjustment
72
In-service checks
72
Maintenance and servicing of the air brake system
72
Air brake adjustment myths
73
Section summary questions
73
Air Brake Manual
1
Section 9 — Pre-trip air brake inspection
Vehicle inspection
Air brake practical test
Pre-trip inspection — combination unit
Brake adjustment — combination unit
Suggested driving practices
Pre-trip inspection — single unit
Brake adjustment — single unit
Suggested driving practices
Pre-trip inspection — air-over-hydraulic
(air actuated) brake system
Suggested driving practices
Section summary questions
75
76
76
77
78
79
80
80
81
Pre-trip inspection tear-out sheets
Metric conversion table
Organ and tissue donation
84
87
89
2
Air Brake Manual
82
82
83
Test appointments
Scheduling your air brake and/or slack
adjustment knowledge or practical test
• All knowledge and practical tests are by appointment
only. Visit any Autopac agent* to pay for your test and
schedule your test appointment.
• Both knowledge and practical tests are conducted during
weekdays only (with the exception of holidays) at the
locations below. Saturday appointments are available
in Winnipeg at the Barnes Street and Lexington Park
locations. For hours of operation please see mpi.mb.ca
or contact an Autopac agent.
• Your Autopac agent can provide you with details of
available appointments at any testing location.
• You can schedule your test appointment up to eight
weeks in advance.
• Knowledge and practical tests are available in English
and French. Specify your language preference when
booking your appointment.
*For a list of Autopac agents, see mpi.mb.ca.
Knowledge test locations
Winnipeg
• 15 Barnes Street (at Bison Drive)
• 125 King Edward Street East
• 40 Lexington Park (at Gateway Road)
• 1284 Main Street
• 1103 Pacific Avenue
• 420 Pembina Highway
• 930 St. Mary’s Road (French Language
Services available)
Outside Winnipeg
• Arborg | 323 Sunset Boulevard
• Ashern | Elks Hall, 2nd Avenue N.
• Beausejour | 848 Park Avenue
• Birtle | Community Hall, 160 Centre Street S.
• Brandon | 731 1st Street
• Churchill | Metis Heritage Hall, 10 Franklin Street
• Cross Lake | Community Council Office, Building 451,
103 Ethan McLeod Dr.
• Dauphin | 217 Industrial Road
• Fisher Branch | Ukrainian National Home,
33 Provencher Street
• Flin Flon | Masonic Lodge, 85 Church Street
• Gillam | Gillam Rec Centre, 235 Mattonnabee Avenue
• Grand Rapids | Town Office, 200 Grand Rapids Drive
• Killarney | Elks Hall, 403 Fletcher Street E.
• Leaf Rapids | Council Office, Town Centre Mall
• Lynn Lake | Corner Pocket Billiards, 467 Sherritt Avenue
• Melita | Legion Hall, 95 Main Street
• Neepawa | Neepawa Public Library, 280 Davidson Street
• Norway House | Kinosao Sipi Multiplex
• Pine Falls | Golden Leisure Club, 30 Pine Street
• Portage la Prairie | 2007 Saskatchewan Avenue W.
• Russell | Russell District Community Centre,
106 Shell River Ave S.
• Selkirk | 1008 Manitoba Avenue
• Snow Lake | Royal Canadian Legion, 102 Elm Street
• Steinbach | 91 North Front Drive
(French Language Services available)
•S
wan River | War Veterans Community Hall, 6th Avenue N.
• The Pas | 424 Fischer Avenue
• Thompson | 53 Commercial Place
• Virden | Legion Hall, 540 8th Avenue S.
• Winkler | 355 Boundary Trail
Practical test locations
Winnipeg
• 15 Barnes Street (at Bison Drive)
• 40 Lexington Park (at Gateway Road)
Outside Winnipeg
See Knowledge test locations.
Mobile Test Units
Mobile Test Units serve rural customers living outside
the above locations. Mobile Test Unit locations and
schedules are available from any Autopac agent or online
at mpi.mb.ca.
Key points about your tests
Cost*
• A knowledge test costs $10.
• Costs for road/practical tests:
• Class 1 licence: $50
• Class 5 licence: $30
• Class 2 or 3 licence: $45 • Class 6 licence: $30
• Class 4 licence: $35
• Air brake: $30**
*Cost may change.
**There is no additional charge for the practical air brake test if you are
taking a road test for a class 1, 2, 3, 4 or 5 licence and the vehicle you are
using for the test is equipped with air brakes.
General testing information
•P
lease check in 15 minutes prior to your scheduled
appointment time.
•T
here is a 30-minute time limit to complete a
knowledge test.
•T
he knowledge test is not an open book test.
•C
ellular phones and electronic devices are not allowed
in the test area.
•O
nly one knowledge or road/practical test of the
same class may be completed per day.
For additional information or
assistance, call your Autopac agent
or the following numbers:
• In Winnipeg call 204–985–7000
• Outside Winnipeg call toll-free 1–800–665–2410
Air Brake Manual
3
Notes
4
Air Brake Manual
Section 1
Brakes and braking
1
Air Brake Manual 5
Air Brake Manual • 5
Brakes and braking
Heat-Energy-Traction-Friction
If a 200-horsepower engine accelerates a vehicle to 100
km/h in one minute, imagine the power needed to stop
this same vehicle. Also, consider that the vehicle might
have to stop in an emergency in as little as six seconds
(just 1/!) the time it took to reach 100 km/h).
For a vehicle to move along the highway, an internal
combustion engine must convert its heat energy into
mechanical energy. This mechanical energy goes from the
engine to the driving wheel tires by means of a system of
connecting rods, shafts and gears. The final factor that
moves the vehicle is the amount of traction its tires have
on the road surface.
100 km/h
10X
Friction is the force that resists movement between two
surfaces in contact with each other. To stop a vehicle,
the brake shoe linings are forced against the machined
surfaces of the brake drums, creating friction. This friction
produces heat.
The engine converts the energy of heat into the energy of
motion; the brakes must convert this energy of motion
back into the energy of heat. The friction between brake
drums and linings generates heat while reducing the
mechanical energy of the revolving brake drums and
wheels. The heat produced is absorbed by the metal brake
drums, which dissipate the heat into the atmosphere.
The amount of heat the brake drums can absorb depends
on the thickness of the metal. When enough friction is
created between the brake lining and the drums, the
wheels stop turning. The final factor that stops the vehicle
is the traction between the tires and the road surface.
To stop the vehicle in 1/!) the time it took to accelerate
would require a stopping force of 10 times the acceleration
force — the equivalent of approximately 2,000
horsepower. If the vehicle had six wheels, each wheel
would have to provide 1/^ the braking force. If one or two
of the wheels had brakes that were not properly adjusted,
the other wheels would have to do more than their share
of the braking, and that might be more than their brakes
were constructed to stand. Excessive use of the brakes
would then result in a buildup of heat greater than the
brake drums could absorb and dissipate. Too much heat
results in brake damage and possible failure.
Most brake linings operate best at around 250°C and
should not exceed 425°C. It’s important to understand
that the power needed to stop generates heat which could
damage the brakes.
Brake drums
250°C
Normal
6
Air Brake Manual
425°C
Maximum
1100°C
Panic!
Speed-Weight-Distance
The distance required to stop a vehicle depends on its
speed and weight, in addition to energy, heat and friction.
The braking force required to stop a vehicle varies directly
with its weight and speed. For example, if the weight is
doubled, the braking force must be doubled to be able
to stop in the same distance. If the speed is doubled, the
braking force must be increased four times to be able to
stop in the same distance. When weight and speed are
both doubled, the braking force must be increased eight
times to be able to stop in the same distance.
For example, a vehicle carrying a load of 14,000 kg at
16 km/h is brought to a stop in 30 metres (m) with normal
application of the brakes. If this same vehicle carried
28,000 kg at 32 km/h, it would require eight times the
braking force to stop the vehicle in 30 m. This would
be more braking force than the brakes could provide.
No vehicle has enough braking force when it exceeds
its limitations.
Delivered force
= 400 lb.
Applied force = 100 lb.
A
C
B
If a 100 lb. downward force is applied at point A,
the upward force at point B is 400 lb.
Braking force
A = 100 lb.
Mechanical
Braking systems use devices to gain a mechanical
advantage. The most common device for this purpose
is leverage.
C
Look at this simple lever system:
B = 400 lb.
4 feet
1 foot
A
B
C
A lever is placed on a pivot called the fulcrum (C). As
the distance from A to C is four feet, and from C to B is
one foot, the ratio is four to one (4:1). Force has been
multiplied by the leverage principle.
Conversely, if a 400 lb. downward force is applied at point
A, the upward force at point B is 100 lb.
S-cam brake
A
100 lb.
C
B
400 lb.
B
400 lb.
Air Brake Manual
7
Use of air pressure
Leverage and air pressure
Force can also be multiplied by the use of air to gain
further mechanical advantage. Everyone has felt the force
of air on a windy day. Air can be compressed (squeezed)
into a much smaller space than it normally would occupy;
for instance, air compressed in tires to support the
weight of a vehicle. The smaller the space into which
air is squeezed, the greater the air’s resistance to being
squeezed. This resistance creates pressure, which is used
to gain mechanical advantage.
In actual operation, pipes are round and plugs are
diaphragms of flexible material acting against pushrods.
If compressed air of 120 psi acts on a diaphragm of
30 square inches, 3,600 lb. of force is produced (120 × 30).
Apply this force to a pushrod to move a 6-inch slack
adjuster operating a cam and the total force equals
21,600 inch pounds torque (3,600 × 6), or 1,800 foot
pounds torque (21,600 ÷ 12). It requires 25 to 30 foot
pounds of torque to tighten the wheel on a car. This
comparison illustrates the force obtained from using
mechanical leverage and air pressure combined.
If a constant supply of compressed air is directed through
a pipe that is one inch square, and if a one inch square
plug were placed in the pipe, the compressed air would
push against the plug. A scale can be used to measure how
many pounds of force are being exerted by the air against
the plug.
30 square inches
120 psi
6 inches
1 inch
1 square inch
10 psi
If the scale registers 10 pounds, for example, then it could
be said the force is 10 pounds on the one square inch
surface of the plug or 10 pounds per square inch (psi).
The more compressed the air in the supply reservoir, the
greater the force exerted on the face of the plug.
Stopping distance
Stopping distance consists of three factors:
• driver’s reaction time
• brake lag
• braking distance
Driver’s reaction time: Reaction time is often called
“thinking time”. The time it takes from the moment a
hazard is recognized to the time the brake is applied is
approximately P of a second.
Brake lag: As air is highly compressible, it requires a
relatively large volume of air to be transmitted from
the reservoir to the brake chamber before there is
enough pressure for the brakes to apply. It can be said
that brake lag is the time it takes the air to travel through
a properly maintained air brake system (approximately
4/!) of a second).
Braking distance: The actual distance the vehicle travels
after the brake is applied until the vehicle stops.
The distance depends on the ability of the brake lining to
produce friction, the brake drums to dissipate heat and the
tires to grip the road.
8
Air Brake Manual
Drivers should never take their brakes for granted. The
braking system must be tested and the adjustment checked
before placing the vehicle into service. Drivers must
understand the braking system, realize its capabilities and
limitations and learn to use them to the best advantage.
Heavy vehicles require powerful braking systems that are
obtained by use of mechanical leverage and air pressure.
Brakes must be used keeping in mind the heat generated
by friction. If the heat becomes too great, braking
effectiveness will be lost. The heavier the load and the
faster the speed, the greater the force needed to stop.
It is important to remember that an air-brake-equipped
vehicle, even with properly adjusted brakes, will not stop
as quickly as a passenger car.
Comparative stopping distances
Passenger
vehicle
Section summary questions
1.What is the final factor that will determine if
the vehicle will move?
2.What is the final factor that will determine if
the vehicle will stop?
3.How is the heat that is generated by the
brakes dissipated?
4.If one set of brake shoes is poorly adjusted, what effect
could it have on the remaining sets of brake shoes in
the system?
5. What is meant by the term “friction”?
6.If the weight of the vehicle is doubled, how many times
must the stopping power be increased?
7.If the speed of the vehicle is doubled, how many times
must the stopping power be increased to be able to
stop at the same distance?
8.If both weight and speed of the vehicle are doubled,
how many times must the stopping power be
increased to stop at the same distance?
Loaded
truck
9. What is compressed air?
10.What does the abbreviation “psi” stand for?
Brakes applied
11.If 40 psi is exerted against a diaphragm of 30 square
inches in area, what are the total pounds of force that
could be exerted?
12.Stopping distance consists of what three factors?
13.Define the following terms:
Actual stop
• Driver’s reaction time
• Braking distance
• Brake lag
Actual stop
Air Brake Manual
9
Notes
10
Air Brake Manual
Section 2
Components of an
air brake system
2
Air Brake Manual
11
Section 1 of this manual explained that it is possible to gain a
mechanical advantage through the use of levers and that air under
pressure can be used to gain a mechanical advantage. Section 2
explains how air under pressure can be used to operate the air
brakes of a vehicle.
Piping illustrations have been kept simple in order to be easily
understood. The piping arrangements found on vehicles in actual
use on the highway might differ somewhat from the illustrations in
this manual.
Components of an
air brake system
A basic air brake system capable of stopping a vehicle has
five main components:
1. a compressor to pump air with a governor to control it
2. a reservoir or tank to store the compressed air
3. a foot valve to regulate the flow of compressed air
from the reservoir when it is needed for braking
4. brake chambers and slack adjusters to transfer
the force exerted by the compressed air to
mechanical linkages
5. brake linings and drums or rotors to create the friction
required to stop the wheels
It is necessary to understand how each of these
components work before studying their functions in
the air brake system.
Compressor and governor
Compressed air is used to transmit force in an air
brake system. The source of the compressed air is
a compressor (1). (Numbers in brackets in this section
refer to the foldout illustration at the front of this manual.)
A compressor is designed to pump air into a reservoir
which results in pressurized air.
The compressor is driven by the vehicle’s engine, either
by belts and pulleys or shafts and gears. In vehicles
where the compressor is driven by belts, they should be
checked regularly for cracks and tension.Also, check the
compressor for broken mounting brackets or loose bolts.
The compressor is in constant drive with the engine.
Whenever the engine is running, so is the compressor.
When pressure in the system is adequate, anywhere from
a low of 80 psi to a high of 135 psi it is not necessary for
the compressor to pump air. A governor (2) controls the
minimum and maximum air pressure in the system by
controlling when the compressor pumps air. This is known
as the “loading” or “unloading” stage. Most compressors
have two cylinders similar to an engine’s cylinders.
When the system pressure reaches its maximum, which
is between 115 and 135 psi, the governor places the
compressor in the “unloading” stage.
The compressor must be able to build reservoir air
pressure from 50 to 90 psi within three minutes. If unable
to do so the compressor requires servicing. A compressor
may not be able to build air pressure from 50 to 90 psi
within three minutes if the air filter is plugged or if the belt
was slipping — if these were not at fault the compressor
could be faulty.
Governor
Pressure
setting
spring
Exhaust port
Unload port
Reservoir port
12
Air Brake Manual
Exhaust port
Unload port
Reservoir port
Placing the compressor in the unloading stage is done by
directing air pressure to the inlet valves of the compressor,
holding them open and allowing the air to be pumped
back and forth between the two cylinders, instead of
compressing the air. When the pressure in the system
drops, the inlet valves close, returning the compressor
to the “loading” stage. The governor must place the
compressor in the “loading” stage at no lower than 80 psi.
During the “unloading” stage, the compressor is able
to cool.
Usually compressors are lubricated from the engine
lubrication system, although some compressors are
self-lubricating and require regular checks of the
lubricant level.
Compressor (unloading stage)
Discharge valve
Inlet valve
It is very important the air that enters the system be kept
as clean as possible. The air must first pass through a filter
to remove any dust particles. The air filter must be cleaned
regularly. A dirty filter will restrict the flow of air into the
compressor, reducing its efficiency. Some vehicles have
the inlet port of the compressor connected to the intake
manifold and receive air that has been filtered by the
engine air cleaner.
A piston-type compressor operates on the same principle
as the intake and compression strokes of an engine.
• I ntake stroke: The downward stroke of the piston creates
a vacuum within the cylinder which causes the inlet
valve to open. This causes atmospheric air to flow past
the inlet valve into the cylinder.
•C
ompression stroke: The upward motion of the
piston compresses the air in the cylinder. The rising
pressure cannot escape past the inlet valve (which
the compressed air has closed). As the piston nears
the top of the stroke, the pressurized air is forced
past the discharge valve and into the discharge line
leading to the reservoir.
Compressor (compression stroke)
Unload plunger
Discharge valve
To reservoir
Inlet valve
Unload plunger
Intake air filter
Piston
From governor
Compressor (intake stroke)
Discharge valve
Intake air filter
Piston
Inlet valve
Unload plunger
Intake air filter
Piston
Air Brake Manual
13
Reservoirs
Reservoirs or tanks hold a supply of compressed air. The
number and size of the reservoirs on a vehicle will depend
on the number of brake chambers and their size, along
with the parking brake configuration. Air brake vehicles
are equipped with more than one reservoir. This gives
the system a larger volume of main reservoir air. The
first reservoir after the compressor is referred to as
the supply or wet (5) reservoir. The other reservoirs
are known as primary (8) and secondary (10) or dry
(8) (10) reservoirs.
When air is compressed, it becomes hot. The heated
air cools in the reservoir, forming condensation. It is in
this reservoir that most of the water is condensed from
the incoming air. If oil leaks past the piston rings of the
compressor and mixes with this moisture, it forms sludge,
which accumulates in the bottom of the reservoir. If
allowed to accumulate, this sludge (water and oil) would
enter the braking system and could cause trouble with
valves and other parts. In winter, water in the system
may freeze, causing the malfunction of valves or brake
chambers. Reservoirs are equipped with drain valves so
that any moisture or sludge that may have accumulated
can be drained. If you notice sludge when draining your
system, have it inspected by a mechanic. To minimize
the amount of water collection, all reservoirs must be
drained daily. Under extreme conditions, reservoirs may
have to be drained more than once a day. To drain the
reservoirs always start with the wet reservoir on the
tractor. Open the drain valve fully and allow all air pressure
to escape, which will also exhaust the moisture collected
in the reservoir.
Some reservoirs may be equipped with automatic
reservoir drain valves (spitter valves). These valves
will automatically exhaust moisture from the reservoir
when required, although they should be checked daily
and drained periodically to ensure the mechanism
is functioning properly. Any loose or disconnected
wires associated with the valve heaters should be
repaired immediately.
Air dryer
An air dryer (3) may be installed between the compressor
and the wet reservoir to help remove moisture from
the compressed air. It may be partially filled with a high
moisture-absorbent desiccant and an oil filter, or it may
be hollow with baffles designed to assist in separating
the moisture from the air. Both types of air dryers use air
pressure to purge or eject the accumulated contaminants
from their desiccant bed. The purge valve has a heater
element, which prevents the moisture from freezing in
cold climate operation. The wiring connected to the heater
should be inspected for loose or disconnected wires. They
are also equipped with a safety valve.
Air dryer (purge cycle)
Control
port
Supply
port
Cut-off
piston
Purge
valve
Delivery
port
Exhaust
Some reservoirs have more than one compartment and
each compartment has its own drain valve, which must be
drained individually. Briefly opening the valve just to allow
some of the air to escape does not drain the moisture! It is
not safe to assume that the wet reservoir, or the presence
of an air dryer, is reason to neglect the other reservoirs
on the power unit, trailers or dollies. They should all be
completely drained daily by opening the drain valve fully
and allowing all of the air to escape. This should be done at
the end of the day.
14
Air Brake Manual
One-way
check valve
Air dryer (Drying cycle)
One-way check valve
Orifice
Desiccant bed
Oil separator
Desiccant
cartridge
Dried air
Air dryer
Supply port
Cut-off
piston
Control port
Check valve
assembly
Reservoir
Compressor
Control
port
Supply
port
Governor
Delivery port
Purge valve
Exhaust
Sump
Heater
element
Safety valve
A safety valve (4) protects reservoirs from becoming over
pressurized and bursting if the governor malfunctioned
and did not place the compressor in the unloading stage.
The valve consists of a spring-loaded ball that will allow
air to exhaust from the reservoir into the atmosphere.
The valve’s pressure setting is determined by the force of
the spring. A safety valve is normally set at 150 psi. If the
pressure in the system rises to approximately 150 psi,
the pressure would force the ball off its seat, allowing the
pressure to exhaust through the exhaust port in the spring
cage. When reservoir pressure is sufficiently reduced to
approximately 135 psi, the spring will force the ball back
onto its seat, sealing off the reservoir pressure. Not all
safety valves have a manual release feature.
Safety valve
Normal air pressure
Air pressure greater than 150 psi
If the safety valve has to relieve pressure, the governor or
compressor requires adjustment, service or repair. This
should be done by a qualified mechanic.
Air Brake Manual
15
Foot valve
The foot-operated valve (31) is the means of applying air
to operate the brakes. The distance the treadle of the foot
valve is depressed by the driver determines the air pressure
that will be applied, but the maximum application will not
exceed the pressure in the reservoir. Releasing the foot
valve treadle releases the brakes.
Treadle
Front brake chambers (32) are usually smaller than
those in the rear because front axles carry less weight.
A brake chamber is usually mounted on the axle, near
the wheel that is to be equipped for braking. Air pressure
is fed through an inlet port. The air pushes against the
diaphragm and the pushrod. The pushrod is connected
by a clevis and pin to a crank arm-type lever called a “slack
adjuster”. This converts the pushing motion of the pushrod
from the brake chamber to a twisting motion of the brake
camshaft and S-cams. When the air is exhausted, the
return spring in the brake chamber returns the diaphragm
and pushrod to the released position.
Brake chamber and slack adjuster (brakes off)
Air inlet
Treadle
spring
Mounting
bolts
Supply from
reservoir
To brake
chambers
When the driver applies the brakes, depressing the
treadle partway, the foot valve will automatically
maintain the application air pressure without the
driver having to adjust the pressure of his foot on
the treadle.
Releasing the treadle allows the application air to
be released through the exhaust ports into the
atmosphere. Air treadles are spring loaded, producing
a different “feel” from hydraulic brake applications.
Pushrod
Diaphragm
Diaphragm
return spring
Brake chamber and slack adjuster (brakes on)
Air inlet
Brake chambers, slack adjusters
and brake lining
A brake chamber (11) (14) (32) is a circular container
divided in the middle by a flexible diaphragm.
Air pressure pushing against the diaphragm causes
it to move away from the pressure, forcing the push
rod outward against the slack adjuster. The force
exerted by this motion depends on air pressure and
diaphragm size. If a leak occurs in the diaphragm,
air is allowed to escape, reducing the effectiveness
of the brake chamber. If the diaphragm is completely
ruptured, brakes become ineffective.
Air Brake Manual
Clevis and pin
Slack
adjuster
Exhaust port
16
Brake chamber
Brake chamber
Mounting
bolts
Clevis and pin
Slack
adjuster
Pushrod
Diaphragm
Diaphragm
return spring
As indicated by its name, the slack adjuster adjusts the
“slack” or free play in the linkage between the pushrod and
the brake shoes. This slack occurs as the brake linings
wear. If the slack adjusters are not adjusted within the
limitations, effective braking is reduced and brake lag time
is increased. If too much slack develops, the diaphragm
will eventually “bottom” in the brake chamber, and the
brakes will not be effective.
Illustrated below are two common types of manual slack
adjusters, showing the worm-adjusting gear.
Manual slack adjusters
Ball indent slack adjuster
Positive lock slack adjuster
Locking collar
Lock screw
Worm shaft
Adjusting bolt
Adjusting bolt
Worm gear
Grease fitting
Spline
On manual slack adjusters, the adjusting worm bolt is
turned until the brake linings touch the drums and then
backed off, normally N to M a turn. A locking device,
which may be a spring-loaded collar over the head of the
adjusting bolt, must be depressed when the wrench is
slipped over the bolt head. This is known as a positive lock
slack adjuster.
Spline
Brake chamber and slack adjuster (brakes on)
Air inlet
Mounting
bolts
Or they could use a spring-loaded internal check ball to
lock the adjustment, and it must be removed to make any
adjustment. This is known as a ball indent slack adjuster.
The more often the driver checks the “slack”, the less the
probability of brake failure. Vehicles rarely “lose” their
brakes because of air loss; it is usually because they are out
of adjustment.
It is the driver’s responsibility to ensure that brakes are
adjusted correctly. A simple service brake application at
low speed to check brake adjustment is not adequate.
Braking at highway speed causes brake drum expansion
due to heat, which in turn requires greater pushrod travel
to maintain the same braking force.
Brake chamber
Clevis and pin
Slack
adjuster
Pushrod
Diaphragm
Diaphragm
return spring
Air Brake Manual
17
If a brake is out of adjustment there would not be enough
reserve stroke of the pushrod travel to compensate for
drum expansion. This would cause a brake fade and greatly
extend stopping distance. If travelling down a hill, this
could cause complete brake loss.
Note: Detailed brake adjustment procedures are outlined in
Section 8.
Some systems have automatic slack adjusters that adjust
automatically to compensate for brake lining wear,
usually maintaining the correct clearance between the
brake lining and drum. Automatic slack adjusters must
be checked regularly to ensure that correct adjustment is
being maintained. There are various makes and models of
automatic slack adjusters in use. Primarily, they are either
stroke-sensing or clearance-sensing. A stroke-sensing
adjuster will adjust the slack when it senses the set stroke
is exceeded. A clearance-sensing adjuster will adjust when
the proper clearance between the brake drum and brake
shoe is not maintained.
Some automatic slack adjusters have the ability to back off
or increase the slack when it has over adjusted the brake.
If a vehicle is equipped with automatic slack adjusters,
it should not be taken for granted that the brakes will
always be in adjustment. The system is not foolproof.
A number of factors could result in the automatic slack
adjuster not maintaining proper slack. There could be
improper installation, inadequate maintenance, deformed
brackets, worn cam bushings, bent pushrods. Even poor
visual inspection can result in problems unrelated to
adjuster function.
When conducting a pre-trip or post-trip air brake
inspection, look for worn or damaged components.
Automatic slack adjusters can malfunction and not keep
the brake in adjustment, especially when it has been in
service for a long period of time. The two most common
problems are excessive premature wear and internal
contamination. Through normal usage, the internal
components that sense when an adjustment is required
wear out. The result is more stroke is required for the
lining to contact the brake drum, and if not checked the
brake could be out of adjustment. If even a small amount
of water is sucked into an automatic slack adjuster
mechanism it can cause corrosion or, in winter, it can
freeze the internal sensing components and inhibit or
prevent adjustment.
Also, under certain conditions, an automatic slack
adjuster that does not have the ability to back off or
increase slack, may over adjust a brake causing it to drag.
For example, this could take place when a tractor-trailer
is negotiating a long, curving downgrade. The driver
should “snub” the brakes, which is repeatedly applying the
brakes moderately to maintain safe control of the vehicle.
Automatic slack adjuster
Clevis pin (small)
Clevis pin (large)
Actuator rod
Clevis
Hairpin clip
Roller (pin)
Boot and strap
Actuator (adjusting sleeve)
Housing
Actuator piston
Pressure relief capscrew (pull pawl)
Thrust washer
Pawl spring
Adjusting pawl
Grease groove
Worm
Grease fitting
Worm seal
Worm gear
18
Air Brake Manual
Adjusting bolt
However, it would not take long in this severe braking
condition for one or more of the brake drums to overheat
and expand. The overheating will physically increase the
brake drums diameter, and in extreme and prolonged
conditions will lead to longer pushrod strokes to achieve
the braking force required.
The automatic slack adjuster interprets this as a need for
adjustment and will take up slack. When the brake drum
cools down and returns to normal size the brakes are over
adjusted and dragging. At that time the driver should
stop and check the brakes for adjustment. A number of
full brake applications above 90 psi per day are required
to keep the automatic slack adjusters in adjustment (see
Section 8 for more information).
Because automatic slack adjusters are not foolproof,
it is important the operator of a vehicle equipped with
automatic slack adjusters be able to manually adjust them.
For information on manually adjusting the automatic
slack adjusters on your vehicle consult the manufacturer.
The illustration below shows a common type of brake
assembly used on truck rear axles and trailer axles.
A front axle assembly has the brake chamber and slack
adjuster mounted on the backing-plate because of the
steering action.
Brake lining material is attached to the shoes. The
material used depends on the braking requirements of the
vehicle. Brake lining must give uniform output of brake
effort with minimum fade at high temperatures.
Fading or reduction in braking effort occurs when
the heated drums expand away from the brake
linings. The brake linings also lose their effectiveness
with overheating.
The twisting action of the brake cam shaft and S-cam
forces the brake shoes and linings against the drums.
The brake linings generate heat from friction with the
brake drum surface.
The thickness of the drums determines the amount of
heat they are able to absorb and dissipate into the
atmosphere. Drums worn thin will build up heat too
quickly. Dangerously undependable brake performance
will result from distorted drums, weak return springs,
improper lining, poor adjustment, or grease or dirt on
the lining. Drums must never be machined or worn beyond
the manufacturer’s specification.
Brake assembly
Brake chamber
Pushrod, clevis and pin
Slack adjuster
S-cam
Brake lining
Brake drum
Air Brake Manual
19
Wedge brakes
Wedge brake — single chamber
This is another example of a brake assembly used on
some air brake-equipped vehicles. The action of the
brake chamber pushrod forces a wedge-shaped pushrod
between the brake shoe rollers. This forces the brake shoe
lining against the brake drum.
Brake
chamber
Brake lining
The vehicle may be equipped with a single or dual
chambers on each wheel, depending on the vehicle’s
size and style.
These brakes may be equipped with a self-adjusting
mechanism or with a manual “star wheel” adjuster.
The star wheel adjustment is made with the vehicle
jacked up, to insure that the brake linings do not drag.
Manual adjustment of wedge brakes is usually done by
a qualified mechanic.
Brake shoe roller
Shoe return spring
Brake shoe
Pushrod
Wedge brake — single chamber
Adjusting wheel
Wedge brake — dual chamber
Brake lining
Brake
chambers
Brake
chamber
Shoe return
springs
Adjusting wheel
20
Air Brake Manual
Adjusting wheel
Disc brakes
The air-activated heavy truck disc brake is similar in
principle to that used on passenger vehicles. Air pressure
acts on a brake chamber and slack adjuster, activating the
brakes. Instead of the cam or wedge used in conventional
heavy truck drum brakes, a “power screw” is used. A power
screw works like a C-clamp, so that the lining pads exert
equal force to both sides of the disc or rotor. Some types
of disc brakes have a built-in automatic adjuster. Disc
brakes that require manual adjustment have adjustment
specifications that differ from conventional S-cam braking
systems. Always check the manufacturer’s specifications
before adjusting. Disc brake assemblies may have a spring
parking brake unit attached to the service brake chamber.
Air-over-hydraulic brake systems
Air-over-hydraulic brake systems were developed for
medium-weight vehicles because:
• Diesel engines do not have a source for vacuum boosting
unless they are equipped with a vacuum pump.
• Medium weight vehicles do not require a full air
brake system.
• It gives the option of pulling an air brake equipped trailer.
These systems combine the best features of an air and
hydraulic brake system. They use hydraulic brakes at
each wheel with their reliable self adjusters and limited
maintenance. On these systems the air is used to either
actuate the hydraulic brakes or boost the hydraulic brake
pressure as explained in the following.
Air Brake Manual
21
Air-actuated hydraulic
brake system
It is essential that the operator of such a vehicle have
knowledge of air pressure build-up time, governor loading
and unloading pressure, warning device operation, and
how to drain air reservoirs properly (see Section 9; Pre-trip
air brake inspection).
(Air brake endorsement required)
An air-actuated system usually has the same components
of a standard air supply system including a warning
buzzer and light, compressor, governor, wet and dry
reservoirs, and a foot valve that could be a single or
dual type. These components are found usually in the
same places as on a full air brake system. Also there are
one or two air-actuated hydraulic pressure converters,
depending on if the system is a single or a dual system.
This system consists of an air chamber or cylinder attached
to a hydraulic master cylinder. When the foot valve is
depressed, the air pressure actuates the pushrod from the
air unit that pushes against the master cylinder piston,
producing hydraulic pressure directed through tubing
to the wheel cylinders actuating the front and rear axle
service brakes.
If an air-actuated hydraulic brake system was to lose its air
supply, the vehicle would have no service brakes. Only the
parking brake would be operating as it is mechanical and
requires no air pressure to operate.
Each vehicle manufacturer may have different parking
brake applications, either automatically when air pressure
is reduced in the reservoir, or mechanically by a brake on
the rear of the transmission, or with the rear brake system.
Since hydraulic brake systems actuated by air pressure are
regarded as an air brake system, your driver’s licence must
have an air brake endorsement for you to operate vehicles
equipped with air-activated hydraulic brakes.
As there are many different systems in use, refer to the
operator’s manual.
Air-actuated hydraulic brake system
Air brake
chamber
Air lines
Foot valve
Hydraulic lines
Hydraulic master
cylinder
Hydraulic
wheel
cylinders
Hydraulic
wheel
cylinders
Hydraulic
master
cylinder
Compressor
22
Air Brake Manual
Reservoirs
Air lines
Air brake chamber
Air-boost hydraulic brake system
(Air brake endorsement not required)
An air-boost hydraulic brake system uses air pressure
to assist brake force. This is similar to vacuum-assisted
brakes on most passenger vehicles. An air-boost system
usually has the same components of a standard air supply
system including a compressor, governor, wet and dry
reservoirs. These components are usually found in the
same places as on a full air brake system. The brake pedal
linkage operates a hydraulic master cylinder that sends
hydraulic pressure to the booster unit. Initially, at low
pressure the hydraulic fluid passes through the booster
and begins to pressurize the wheel cylinders moving the
brake shoes out to the drums. These booster units are
similar in operation to “Hypower” or “Hydrovac” vacuum
boosters found on most light and medium weight vehicles,
but air pressure is used to intensify the hydraulic pressure
generated by the master cylinder rather than vacuum.
Built into the booster unit is a hydraulically-operated air
control valve.
This is where air from the reservoir is directed. As the
pressure from the master cylinder increases, the air
control section in the booster will open and begin to
deliver air pressure to the rear of the air cylinder. The air
cylinder pushrod transfers pressure on a piston in the
hydraulic section of the booster, increasing the hydraulic
pressure at the wheel cylinders.
The driver has full control of the braking force as the
air control section modulates the boost pressure in
proportion to the master cylinder pressure. If the vehicle
was to lose all of the air pressure the brake system would
lose the air assist boost; however, the hydraulic system
would continue to work but at reduced effectiveness.
An air brake endorsement on a driver’s licence is not
required to operate a vehicle with this brake system.
Consult the operator’s manual for the vehicle you drive
for maintenance requirements.
Air-boost hydraulic brake system
Air lines
Booster unit
Hydraulic line
Hydraulic master
cylinder
Hydraulic
wheel
cylinders
Hydraulic
wheel
cylinders
Brake pedal
Compressor
Reservoirs
Air lines
Booster
unit
Hydralic line
Air Brake Manual
23
Section summary questions
1.What are the five basic components of an air
brake system?
2.At what pressure should the governor cause the
compressor to return to its “loading” stage?
3.At what pressure will the governor place the
compressor in the “unloading” stage?
4.How is a plugged air filter likely to affect the
air compressor?
5. What causes moisture to form in the air brake system?
6.When is the compressor able to accomplish most of
its cooling?
7. How are most compressors lubricated?
8. How often should the reservoirs be drained?
9.Is it necessary to allow all the pressure to escape from
the reservoir in order to remove the moisture and
sludge which may have accumulated?
10.What is the maximum pressure available for a full brake
application at any given time?
11.What will result if the brake drums are worn thin or
turned too far?
12.If the governor valve failed to “unload” the compressor,
what would protect the reservoirs from becoming over
pressurized and bursting?
13.What is the purpose of having more than one reservoir?
14.What are two functions of the slack adjusters?
15.Does the amount of slack in the brake linkages have
any effect on the braking efficiency of the vehicle?
16.What is the advantage of keeping the brake chamber
pushrod travel adjusted within limitations?
17.What is the most common cause of loss of effective
braking in an air brake system?
18.Do automatic slack adjusters on S-cam brakes
require checking?
19.Can the adjustment on air-operated disc brakes differ
from S-cam brakes?
20.What occurs when drum brakes become overheated?
21.What causes brake fade?
22.What is the main function of the foot valve?
23.Why does the “feel” of an air-operated foot valve differ
from a hydraulic brake pedal?
24.On what principle does a disc brake operate?
25.What type of air-over-hydraulic brake system requires
the operator to hold an air brake endorsement?
24
Air Brake Manual
Section 3
How the basic air
brake system works
3
Air Brake Manual
25
How the basic air
brake system works
Basic air brake system
The following explains the additional components of a
basic air brake system. Other valves which are necessary
to ensure smooth and efficient operations are not included
in this simple drawing. They will be discussed later in the
manual.
Air is pumped by the compressor (1) to the wet reservoir
(5), which is protected from over pressurization by a
safety valve (4). The governor (2) controls the pressure
in the reservoir to the bottom of the foot valve (31). The
driver pushes the foot valve treadle down and air pressure
flows to the front and rear brake chambers (32) (11). The
brake chamber pushrods move the slack adjusters. The
slack adjusters rotate the S-cams, forcing the brake shoes
against the drums. This causes friction that stops the
wheels. The driver releases the foot valve treadle and the
air in the brake chambers is allowed to exhaust through
the foot valve, releasing the brakes.
Note: An air dryer (3) has been added to reduce the amount of
moisture in the system.
Basic air brake system
32
31
11
2
32
1
3
4
5
One-way check valve
In this illustration, two reservoirs are shown (5) (10) and
10). To prevent air from flowing backwards in the system
toward the compressor, a one-way check valve (7) is
installed between the reservoirs. This valve allows the air
to flow in one direction only. The valve is spring loaded.
Pressure at the inlet side overcomes the spring pressure
and lifts the check valve ball, or disc, off its seat. Air passes
through the valve to the outlet. When pressure at the
outlet becomes greater than at the inlet — together with
the spring pressure — the check device seats, preventing
air from flowing back through the valve.
7
9
10
11
One-way check valve
Spring
Body
Ball
Cap nut
26
Air Brake Manual
Air pressure gauge
Vehicles with an air brake system are equipped with a
reservoir air pressure gauge (29). This gauge is mounted
in the cab, usually on the dashboard and indicates the air
pressure in the primary and secondary or dry reservoirs.
The supply or wet reservoir does not usually have an air
pressure gauge. Common operating pressures are 80 to
135 psi, depending on the system. Monitoring the gauge
will alert the driver to any unusual changes in air pressure.
Wig-wags are not found in modern vehicles, having been
replaced with a red warning light and buzzer. They may
still be in use on older vehicles. There are two types of
wig-wag low pressure warning devices that may be used.
Both types will drop into the driver’s view should the
system pressure drop to 60 psi. The automatic warning
device will rise out of the driver’s view when the pressure
in the system rises above 60 psi. The manual reset type
must be placed in the “out of view” position manually and
will not stay in place until the pressure in the system goes
above 60 psi.
Whichever warning system is used, buzzer-lights or
wig-wag, the driver must stop the vehicle in a safe place
immediately and find the cause of the air loss. The air
pressure remaining in the system (approximately 60 psi) is
enough for a brake application if the driver acts promptly.
Keep in mind if the vehicle has spring parking brakes, at
60 psi the spring parking brakes are partially applied and
you cannot release them until air pressure increases. If
you do not move your vehicle to a safe place you may find
yourself stranded in the traveled portion of the highway.
Stop light switch
Brake application gauge
An additional gauge can be installed on the dash to
indicate the application air pressure when the brakes are
applied. This gauge can be piped to indicate the pressure
of either a foot or hand application. (Hand application will
be explained later in the manual.)
Any driver following your vehicle must be warned when
reducing speed or stopping the vehicle. The stop light
switch (25) is an air-operated electric switch that turns
on the brake lights at the rear of the vehicle when a brake
application is being made.
Quick release valve
The application of the brakes in the basic system was
described earlier. In a basic system, when the driver
releases the foot valve, it would be necessary for the air
under pressure in the brake chambers to return to the foot
valve to release the brakes. This releasing action would be
slowed in long wheel base vehicles because of the longer
lines between the foot valve and the rear brake chambers.
A quick release valve (33) is installed to allow the brakes to
release quickly and fully by discharging the application air
near the brake chambers.
Low pressure warning device
All vehicles equipped with an air brake system must have
a device to warn the driver if the air pressure in the system
drops to a dangerous level. This device could be a red
warning light, a buzzer or a wig-wag. Due to overuse or
leaks, the low pressure indicator switch (9) will turn on a
red warning light on the dash or cause a buzzer to sound
at or before 60 psi. Some vehicles are equipped with
both a light and a buzzer to warn the driver of a low air
pressure condition.
Air Brake Manual
27
Relay valve
Manual front brake limiting valve
The foot valve is usually located closer to the front wheels
than to the rear wheels. The longer the distance from the
foot valve to the rear chambers, the more time it will take
before the rear brakes apply. This is known as brake lag.
To correct this condition on a long wheel base vehicle, a
relay valve (13) is installed near the rear brake chambers.
A larger diameter pipe is connected between the main
reservoir and the relay valve. The air line from the foot
valve to the relay valve now becomes a “control line”. (The
air in the control line “dead ends” at the relay valve.) When
the foot valve is depressed, the air pressure in the control
line acts on the top section of the relay valve, relaying
reservoir air directly to the rear brake chambers through
the larger diameter pipe. The pressure of the reservoir
air delivered in this way will be the same as the control
pressure delivered by the foot valve.
For better steering control when braking on a slippery
road surface, it can be an advantage to reduce the braking
effort to the front wheels. This can be accomplished by
installing a control valve (35) in the cab and a front brake
limiting valve (36) on the front axle.
The control valve is set in the “normal” position for dry road
surfaces and the front braking application air pressure is
normal. On a slippery road surface, the control valve (35)
is set to the “slippery road” position. In this position, the
control valve will cause the limiting valve (36) to operate.
Applying air pressure to the front brakes is then reduced to
50 percent of the application air pressure being delivered
to the rear brake chambers.
Manual front brake
limiting valve
Releasing the foot valve exhausts the control air to the
relay valve, allowing it to cut off the flow of reservoir
air to the rear chambers. This in turn exhausts the air in
the brake chambers by the quick release feature of the
relay valve.
Dash mounted
control valve
Service port
Lever
Relay valve
From
limiting
valve
To limiting
valve
Exhaust port
Service port
(Delivery ports not shown)
Basic air brake system with manual front brake limiting valve
29
32
11
35
31
13
36
32
1
3
2
28
Air Brake Manual
4
6
5
7
6
8
9
11
Automatic front brake
limiting valve
Some systems are equipped with an automatic limiting
valve (34). This valve will hold off brake application to the
front wheels from 0 to 10 psi, depending on how it has
been preset. Between the preset pressure and 40 psi of
brake application, the reduction is approximately 50 per
cent. Brake applications between 40 psi and 60 psi are
reduced by less than 50 per cent. Brake applications more
than 60 psi are not reduced and full application is directed
to the front wheels.
Automatic front brake limiting valve
Service port
Service port
Piston spring
Inlet-exhaust
valve spring
Lower piston
assembly
Delivery port
Delivery port
Tandem rear axles
The air brake system discussed previously is for a vehicle
with a single rear axle. The illustration below shows an
air brake system for a vehicle equipped with an automatic
front brake limiting valve (34), a quick release valve (33)
and a tandem set of rear axles. Both axles of the tandem
set are equipped with brakes.
Tandem rear axles
33
A relay valve (13) has two uses: to provide a quicker
application of air pressure to the tandem rear axle brakes
when a brake application is made, and to release the
brakes quicker when a brake application is released.
34
13
Air Brake Manual
29
Section summary questions
1.How can the driver tell how much air pressure is in the
main reservoirs?
2.What must the driver do when a low pressure warning
system activates?
3. What is the purpose of a quick release valve?
4. What is the purpose of a relay valve?
5.What is the purpose of using a larger diameter pipe
between the reservoir and the relay valve?
6.If the front brake limiting valve is in the “slippery
road” position, and the foot valve is depressed to
make a brake application of 30 psi, how much pressure
will be applied in the front brake chambers?
7.How is the reservoir protected from
over pressurization?
8.What stops pressurized air from flowing from the dry
reservoir back into the compressor?
9.At what pressure should the low pressure warning
device activate?
10.How is “brake lag” to the rear wheels minimized?
11.When should a driver use the front brake limiting valve?
30
Air Brake Manual
Section 4
Spring parking
brake systems
4
Air Brake Manual
31
Spring parking
brake systems
(Single circuit system only)
The installation of spring parking brakes and their piping
arrangements into a vehicle air brake system will vary
depending on the vehicle make.
Spring parking brakes may be installed on an air brakeequipped vehicle for use as a reliable parking brake system.
In the service brake system, the brakes are applied by air
pressure and retracted by springs. In the spring parking
brake system, the brakes are applied by spring pressure
and retracted by air pressure. The spring parking brake
chambers are attached to the service brake chambers
and operate through the same linkage, therefore the
effectiveness of the spring parking brake depends on
the service brake adjustment. The spring parking brake
control valve (operated by a square, yellow button)
located in the cab allows the driver to exhaust air out of
the spring parking brake circuit to apply the brakes, or
pressurize the circuit to release them. Some systems may
have an additional valve controlled by a blue button that
applies only the tractor spring parking brakes and not
the trailer spring parking brakes. The system can also act
as an emergency brake. Loss of air from the system may
automatically apply the brakes.
A spring-loaded valve requires that the valve be pushed
in to release the spring parking brakes. This valve cannot
be left in the released position with air pressure in the
system below approximately 35 psi. Any time the reservoir
pressure drops to approximately 35 psi, this valve will
close automatically, placing the spring parking brakes
into full application. On some older vehicles there may
be a single type of push-pull control valve that does not
have an automatic release feature. To apply the spring
parking brakes, the valve must be operated manually,
even though the reservoir pressure has been depleted.
During normal operation, air pressure cages
(compresses) the spring, holding it ready for parking
or emergency braking.
Brakes off
Service brake
chamber
Parking brake
spring
Dust
cap
Mounting bolts
Clevis and pin
Control valves will vary, depending on the manufacturer
and type of piping arrangements.
Slack
adjuster
Pushrod
Spring parking brake
32
Air Brake Manual
Spring
parking
brake
chamber
Diaphragm
return
spring
Diaphragm
On the pre-trip air brake inspection (Section 9), you must
ensure that the parking brake spring is not manually
caged or it will not expand and apply the brake. The brake
chambers should be checked for cracks and damage. The
brake chamber should be fitted with a dust cap to ensure
debris will not enter the chamber.
During normal service brake operation, the parking
brake spring does not expand. Air pressure keeps the
spring caged.
However, it takes approximately 90 psi of air pressure to
keep the springs caged. If reservoir air pressure is allowed
to drop below 90 psi the springs will start to move. At
approximately 70 psi the brakes will be partially applied
making it difficult to continue driving. If the air pressure
drops below 60 psi the brakes will be fully applied making
it impossible to move the vehicle until air pressure is built
up to recage the spring parking brakes or the springs are
manually caged.
Using a spring parking brake
Spring parking brakes (12), added to the brake chambers
of the rear axle on the single unit vehicle, are illustrated.
A control valve (27) is mounted in the cab. A supply line
of reservoir air is piped from the dry reservoir to the
control valve. Opening the control valve allows reservoir
air pressure to flow to the spring parking brake chambers,
releasing them.
Service brakes applied­ — brakes on
Service brake
chamber
Parking brake
spring
Dust
cap
Mounting bolts
Clevis and pin
Slack
adjuster
Pushrod
Spring
parking
brake
chamber
Diaphragm
return
spring
Diaphragm
Closing the control valve shuts off the supply of reservoir
air pressure and exhausts the existing pressure in the
spring parking brake chambers. This motion allows the
spring to expand, applying the brakes.
Caution: Parking brakes should be in the release position
before making a service brake application. A full-brake
application, made when the parking brakes are applied,
can compound the force exerted on the slack adjusters
and linkage and result in damage or brake failure.
Compounding is the combination of two forces: the
force applied by the spring brake chamber and the service
brake chamber.
Using a spring parking brake
27
12
12
Air Brake Manual
33
Parking brake spring caged­ — brakes off
Spring parking brakes applied­ — brakes on
Service brake
chamber
Dust
cap
Mounting bolts
Clevis and pin
Spring
parking
brake
chamber
Diaphragm
return
spring
Diaphragm
Spring brakes are primarily used as a parking brake, but
in the event of loss of air pressure in the system, they can
assist in stopping the vehicle. How quickly they will stop
the vehicle depends on such factors as:
• the weight and speed of the vehicle
• the steepness of the grade
• the spring force of the spring brakes that have
been installed
• the adjustment of the service brakes
If the brakes have overheated, such as during mountain
driving or hard highway braking, care must be taken
when parking the vehicle. If the spring parking brakes are
applied when the brake drum has expanded because of
extreme heating, when the brake drum starts to cool and
contract, the pressure exerted by the spring parking brake
may cause the brake drum to crack or warp. When parking
a vehicle with overheated brakes, park on level ground,
stop the engine and leave the transmission in the lowest
gear and block the wheels. Do not set the spring parking
brakes until you have verified the brake drum is cool to
the touch.
Mechanical release (caging)
Some spring parking brakes can be released mechanically
by “winding them off” or “caging” them. Caging means the
brakes are being released. This is achieved with a bolt that
runs through the centre of the chamber body, which is
turned to compress the spring. It may be necessary to first
remove a lock plate and stud to gain access to the head
of the bolt. Other types have a dust cap that must first
be removed and a bolt inserted. In some cases, a special
wrench is required.
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Air Brake Manual
Parking brake
spring
Mounting bolts
Caging
bolt
Clevis and pin
Slack
adjuster
Pushrod
Service brake
chamber
Parking brake
spring
Slack
adjuster
Pushrod
Spring
parking
brake
chamber
Diaphragm
return
spring
Diaphragm
Instruction on how to “cage” is usually on the body of the
parking brake chamber. If all air is lost and the vehicle has
to be towed, the parking brakes can be released by caging
them. Always block the wheels when caging the parking
brake spring.
Warning: Spring parking brake chambers should never be
disassembled without first compressing the spring with
a caging bolt. These springs are under extreme pressure
and could cause serious personal injury if disassembly is
attempted by anyone not experienced in servicing these
units. Disassembly of a spring brake chamber should only
be preformed by a qualified mechanic or technician.
Section summary questions
1. What is meant by “compounding” the brakes?
2. Why are spring brakes a reliable type of parking brake?
3. How are parking brakes held in the released position?
4.What are the functions of the cab-mounted parking
brake control valve?
5.Will parking brakes apply “automatically” in all
braking systems?
6.What is the reason for releasing the parking brakes
before making a full brake application test?
7.Why must you be careful parking a vehicle with
overheated brakes?
8.How can some types of parking brakes be released
without the use of air pressure?
9.What is the danger of disassembling a spring
parking brake unit?
Section 5
Trailer system
5
Air Brake Manual
35
Trailer system
Up to this point, the system discussed is the air brake
system of a truck or tractor. If a trailer was coupled
to a truck or tractor, the brakes of the trailer would have
to be operated from the truck or tractor.
If the unit is not equipped with “dead-end” couplers, the
glad hand of the control (service) line can be locked to the
glad hand of the supply (emergency) line to keep water
and dirt from entering the lines.
In the following pages the power unit of a combination
vehicle will be referred to as a tractor.
The cleaner the air supply is kept, the less chance of
brake problems.
Glad hands
Glad hands and lines should also be secured to prevent
the lines from chafing against vehicle components or
bouncing off the vehicle. This could seriously damage the
glad hands or lines.
This term refers to the coupling device used to connect the
control (service) and supply (emergency) lines of the trailer
to the tractor. These couplers connect together and lock in
position. They have rubber gaskets that prevent air from
escaping at the connections.
Air line
Application line
The application line is referred to as a control (service) line.
This line is connected to the foot and hand valve. When
the driver depresses the foot valve treadle application
air will be delivered to the tractor brake chambers and to
the trailer brake chambers. When the driver releases the
foot valve treadle, the application air to the trailer brake
chambers must return to the foot valve to be exhausted to
the atmosphere.
The disadvantages of this system are:
• I f the trailer broke away from the tractor, the trailer
would not have brakes.
• I f the control (service) line parted or ruptured, the trailer
brakes would not be applied, and the application air
would be lost from the tractor if the brakes were applied.
• I f the air pressure in the reservoirs is lost, there would be
no way to apply the brakes of the tractor or the trailer.
Rubber seal
•T
he trailer brakes cannot be applied independently from
the tractor and there is no way to set the trailer brakes
when coupling to the tractor.
•T
he application and release of the trailer brakes would be
slower than those of the tractor.
Air line
Before connection is made, couplers should be clean and
free of dirt and grit. When connecting the glad hands,
start with the two gaskets together and the couplers at a
90° angle to each other. A quick snap downwards will join
and lock the couplers. “Dead‑end” couplers should be used
whenever the vehicle is driven without a trailer to prevent
water and dirt from entering the coupler and lines.
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Air Brake Manual
These disadvantages are overcome by the addition of
the supply (emergency) line and valves discussed in the
following pages.
The illustration below shows the piping of a unit with
brakes applied, similar to the tandem axles of the tractor.
Also with brakes applied, the trailer has tandem axles
equipped with brake chambers.
The application line has a “T” inserted between the foot
valve (31) and the tractor’s relay valve (13). An air line
has been connected from this “T” to the trailer by a set of
couplers (glad hands) (20).
Application line
“T”
13
20
31
Trailer brake hand valve
The purpose of the trailer brake hand valve (30) is to
allow the driver to control independently the amount
of application air pressure to be directed to the trailer
brakes. It also provides a method of applying the trailer
brakes when coupling the trailer to the tractor.
The valve also allows the driver to apply the trailer
brakes independently of the tractor. The amount of
application air pressure delivered depends on the amount
the valve is opened by the driver. (It cannot exceed the
reservoir air pressure.) Some valves are equipped with
self-returning handles.
Note: The trailer brake hand valve
is not to be used for parking, as
air may bleed off if the engine is
stopped or the hand valve moves
to the released position.
Trailer brake hand valve
30
31
Air Brake Manual
37
Two-way check valves
Two-way check valves
The purpose of a two-way check valve (26) is to direct
air flow into a common line from either of two sources.
This valve will permit air to flow from the source that is
supplying the higher application pressures. The shuttle
will shift so that the higher pressure will be directed to
the trailer through the control (service) line. This valve is
located between the foot-operated valve and the handoperated valve.
Foot valve application
The driver has applied the brakes by using the foot valve
(31). Application air is directed to the brake chambers
of the tractor and to the trailer brakes through a twoway check valve (26). The shuttle has shifted to the low
pressure side, closing off any air flow toward the hand
valve side. The hand valve (30) is in the closed position
and equal pressure is being applied to the brake chambers
of the tractor and the brake chambers of the trailer.
Air from
hand valve
Air from
foot valve
Shuttle
To trailer
Foot valve application
30
26
31
Hand valve application
In this illustration with the foot valve (31) released and
the hand valve (30) opened, application air is directed
from the hand valve through a two-way check valve (26)
to the brake chambers. The two-way check valve in this
application has shifted to the low pressure side, closing off
any air flow toward the foot valve side.
Any time a trailer brake application is made using the hand
valve, the driver may depress the foot valve treadle. If the
foot valve application is of a higher pressure than that of
the hand valve, the two-way check valve will shift to the
lower pressure side, allowing the higher pressure to be
directed to the tractor and trailer brakes.
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Air Brake Manual
During a foot valve application, if the driver makes a hand
valve application of a higher air pressure, the two-way
check valve will direct the higher hand valve air pressure to
the trailer brakes.
Although the trailer brakes may be applied independently
by means of the hand valve, the maximum application
pressure can only be the same as, or slightly less than,
reservoir pressure.
Hand valve application
30
26
31
Tractor protection system
A tractor protection system prevents total loss of air from
the tractor if the trailer breaks away, or if the connecting
air lines between tractor and trailer become separated or
ruptured. The tractor protection system consists of two
valves: the tractor protection valve and the trailer supply
valve. Other names for the trailer supply valve are “trailer
supply valve” and “emergency valve.”
There are two types of trailer supply valves. The most
common is the automatic trailer supply valve. This is a
spring-loaded valve that is opened manually and held open
by air system pressure. On older vehicles you may find a
manual trailer supply valve, which may be a toggle-type
switch or a push/pull-type valve.
To understand the function of the trailer supply valve and
the tractor protection valve in the system, it is important
to understand how they operate.
Disconnecting the supply (emergency) line between the
tractor and trailer while the trailer is charged will cause
an immediate loss of pressure in the supply (emergency)
line. The sudden loss of air pressure in the line will cause
the trailer supply valve to close, and with no air pressure in
the line the tractor protection valve will close. When the
tractor protection value closes it will stop the air flow to
the control (service) line.
In the event of a control (service) line rupture or
disconnection between the tractor and trailer, no action
or air loss will take place until a brake application is
made. Service air will be lost out of the disconnected or
ruptured line causing the tractor’s air pressure to drop.
At approximately 45 psi the trailer supply valve will close
causing an emergency application of the trailer brakes and
the tractor protection valve to close. This will stop the loss
of service air at the disconnected line.
Tractor protection valve
A tractor protection valve (24) is normally located near the
rear of the tractor cab and has two air lines running to it,
one from the trailer supply valve (28) and the other from
the two-way check valve (26) fed by the hand or foot valve.
It also has two air lines: one going to the supply
(emergency) line (21), and one going to the control
(service) line (22). The tractor protection valve is springloaded and requires approximately 45 psi in the supply
(emergency) line to open the valve. This allows control
(service) air to pass through the control (service) line to
the trailer while making a brake application. When air
lines from the tractor are coupled to a trailer, the opening
or closing of the trailer supply valve opens or closes the
tractor protection valve.
To the supply
(emergency)
glad hand
To the control
(service) glad hand
Air Brake Manual
39
The tractor protection valve also protects the tractor’s
service air from being lost during normal brake
applications while operating the tractor without a trailer.
To test the proper operation of the tractor protection
valve, ensure the tractor spring parking brakes are
applied and the wheels are blocked. Verify the system
is at full pressure and the trailer supply valve is open
(trailer charged). Exit the cab and disconnect the control
(service) and ensure that there is no air loss from the
control (service) line glad hand. Enter the cab, ensure full
reservoir pressure between 115 psi and 135 psi and open
the tractor spring parking brake control valve. Shut off
the engine and make a full service brake application. Air
should escape from the control (service) line to no lower
than 20 psi, the tractor protection valve will close and the
air loss will stop. Release the service brake application.
Close the tractor spring parking brake control valve and
ensure the trailer supply valve is closed, if not, close
manually. Start the engine and rebuild air pressure. With
air pressure at or above 90 psi, make a trailer service brake
application with trailer hand valve. No air should escape
from the disconnected control (service) line. Release the
brake application. Exit the cab and re-connect the control
(service) line glad hand.
This tests the proper opening and closing operation of the
spring portion of the tractor protection valve. A tractor
protection valve with a broken return spring will not close
the valve and will cause loss of air during normal brake
applications if operating the tractor without a trailer.
Tractor protection system (open) (trailer charged)
Trailer supply valve
(mounted in cab)
(28)
Supply
(emergency)
line
(21)
Air from
reservoir
Air from
hand valve
Control
(service)
line
(22)
Two-way
check valve
(26)
Air from
foot valve
40
Air Brake Manual
Tractor protection valve
(24)
Tractor protection system (closed) (trailer not charged)
Trailer supply valve
(mounted in cab)
(28)
Supply
(emergency)
line
(21)
Air from
reservoir
Air from
hand valve
Control
(service)
line
(22)
Two-way
check valve
(26)
Air from
foot valve
Tractor protection valve
(24)
Trailer supply valve
The trailer supply valve (28) (usually a red octagonal
button) is mounted in the cab of the tractor. The driver
opens the valve by pushing or pulling the button,
depending on the type used.
Opening the valve permits reservoir air pressure to
flow through. This air pressure is piped to the tractor
protection valve and then to the supply (emergency) line.
The valve is spring-loaded and will be held in the open
position when sufficient pressure is reached. If the air
pressure drops to a range between 45 and 20 psi, the valve
will close automatically by spring pressure. The driver
can close the valve manually, applying the trailer spring
parking brakes.
Air Brake Manual
41
Automatic trailer supply
valve system
The illustration below shows air being piped from the dry
reservoir line to the trailer supply valve (28). The tractor
protection valve (24) is fed by two lines: one from the
trailer supply valve (28) and one from the two-way check
valve (26). Leading off from the tractor protection valve
are two lines, each with a glad hand coupler (20). These
two lines are referred to as the control (service) line (22)
and the supply (emergency) line (21).
In this example, the trailer is not coupled and the tractor
is being operated alone (“bobtailing”). The driver has not
opened the trailer supply valve (28) and the hand valve
(30) is closed.
Automatic trailer supply valve system
30
28
26
22
21
24
20
31
42
Air Brake Manual
In this example, the driver has made a brake application
with the foot valve (31) and application air is being
delivered to the tractor brake chambers. The two-way
check valve (26) has shifted to the low pressure side,
allowing application air to reach the closed tractor
protection valve (24). There is no air loss from the tractor
through the disconnected glad hands (20).
Foot valve applied
30
28
26
24
20
31
If the driver, by mistake, applied the hand valve (30) with
the trailer disconnected, the application air directed to the
tractor protection valve would also be dead-ended. Again,
no air loss would occur if the trailer supply valve (28) is in
the closed position.
Hand valve applied with trailer disconnected
30
28
26
24
20
31
Air Brake Manual
43
Tractor and trailer coupled
In this illustration, the trailer has been coupled to the
tractor, and the control (service) and supply (emergency)
lines have been coupled using glad hands (20).
The trailer unit has a reservoir (16) installed. This
reservoir will provide a volume of air near the trailer brake
chambers for normal or emergency braking. The trailer
reservoir is fitted with a drain valve (6), the same as a
tractor reservoir.
A relay emergency valve (39) is mounted on the trailer
reservoir or to the trailer frame near the brake chambers.
The relay emergency valve serves three main functions in
the system:
1. I t relays air from the trailer reservoir to the trailer brake
chambers during a brake application. This part of the
valve operates like the relay valve previously discussed.
It also provides a quick release of the trailer brakes.
2. It directs trailer reservoir air pressure to the trailer
brakes, causing an emergency application of the brakes.
This action will occur automatically in the event of
ruptured or separated supply (emergency) air line
between the tractor and trailer, or loss of air pressure
from the tractor. A ruptured control (service) line would
not cause an emergency brake application. If a brake
application was made there would be a rapid loss of
tractor reservoir air. If the brake application was held
the air pressure would drop low enough to cause an
emergency application. The driver at any time may
operate the cab-mounted trailer supply valve (28) to
cause an emergency application of the trailer brakes.
3. It has a one-way check valve which stops air pressure in
the trailer reservoir from going back to the tractor.
Tractor and trailer coupled
28
20
6
16
39
44
Air Brake Manual
Charging the trailer system
In the illustration, the compressor has raised the reservoir
air pressure to maximum.
The driver has opened the trailer supply valve (28) to
allow the reservoir air pressure to be directed through the
tractor protection valve (24) to the trailer. The air pressure
passes through the relay emergency valve (39) to the
reservoir on the trailer. Pressure will build up in the trailer
reservoir to the same pressure as the reservoirs on the
tractor. This is known as “charging” the trailer system. The
trailer supply valve should only be open when the tractor
pressure has built up to approximately 90 psi, depending
on the valve.
Charging the trailer system
28
24
39
Foot or hand valve
brake application
This and the next illustration are only to show which brake
components and lines are used for a foot valve and hand
valve application.
• application with the foot valve (31) in orange
• application with the hand valve (30) in brown
Orange and brown are used to show where air is applied
and does not represent the source of the air. When the
foot valve is depressed, the application air will be applied
to the tractor brakes and the trailer brakes together. As
previously explained, the two-way check valve (26) has
shifted, and application air is being directed through the
tractor protection valve (24) to the control (service) line.
If the foot valve is released and the hand valve is applied,
the shuttle in the two-way check valve would shift and
application air would apply the trailer brakes only.
Foot valve brake application
30
26
24
31
Air Brake Manual
45
Application air directed from either the foot or hand valve
causes control pressure to move through the control
(service) line and act on the relay emergency valve (39).
This control pressure will cause the relay emergency valve
to direct air from the trailer reservoir (16) to the trailer
brake chambers (14). The pressure directed by the relay
emergency valve to the trailer brake chambers will be
the same as the pressure directed to the tractor brake
chambers. In such a system, brake lag time has been
minimized with the addition of the trailer reservoir and
relay emergency valve.
Release of the foot or hand valve stops the flow of
application air. The relay portion of the valve returns to
its original position, stopping the flow of air pressure.
The exhausting ports of the valve exhaust the air pressure
from the brake chambers, releasing the brakes. In this
system, the brakes of both the tractor and trailer can be
released quickly.
Caution: Trailer brakes must not be used to hold a parked
vehicle that is left unattended. Loss of pressure will result
in loss of brakes. Always set the parking brake.
Hand valve brake application
30
14
31
16
39
46
Air Brake Manual
Emergency application
If a trailer that is not equipped with spring parking brakes
were to separate from the tractor, the control (service) line
(22) and the supply (emergency) line (21) would detach
from the tractor. The sudden loss of air pressure in the
supply (emergency) line will trigger the relay emergency
valve (39) which causes the trailer reservoir (16) to dump
its air directly to the trailer brake chambers (14). This
places the trailer brakes into emergency application.
The check valve in the relay emergency valve will close,
preventing any bleed back of pressure from the trailer
reservoir. The loss of pressure in the trailer supply
(emergency) line will cause the tractor protection system
to seal off the tractor leaving sufficient air pressure to
brake the tractor.
The trailer brakes will remain applied until either the
pressure in the trailer reservoir and lines is drained off,
or the supply (emergency) line is repaired and the system
is recharged.
Emergency application
30
14
31
16
39
Air Brake Manual
47
Supply (emergency) line rupture
A rupture of the supply (emergency) line (21) or an
uncoupling of the supply (emergency) line glad hands
will trigger the same action as described above.
Supply (emergency) line rupture
26
14
16
39
Control (service) line rupture
It should be noted that any problem that causes a severe
drop in reservoir pressure on the tractor system will cause
the low warning device to signal the driver.
If the control (service) line (22) is ruptured or
disconnected, no action will take place until a brake
application is made. When it is made by either the foot or
hand valve, the air loss from the control (service) line will
lower the air pressure in the tractor’s reservoirs quickly,
depending on the amount of braking demanded. This air
loss will eventually activate the tractor protection system
which in turn will exhaust the trailer supply (emergency)
line and cause the trailer relay emergency valve to apply
the trailer brakes.
In the following illustration, the control (service) line (22)
has ruptured and the driver is making a brake application
with the foot valve (31). The tractor will have brakes
applied but the trailer will have no brakes. If the brake
application is held, the air pressure in the tractor system
will lower to a dangerously low level and then the tractor
protection system will place the trailer brakes into an
emergency application.
Control (service) line rupture
22
31
48
Air Brake Manual
Loss of reservoir air pressure
There is sufficient reservoir air pressure in the primary/dry
reservoir for a limited number of brake applications to stop
the vehicle before the spring parking brakes are activated.
(This will depend on how the spring parking brakes are
piped in the system.)
Rupture of the compressor discharge line would result
in loss of air pressure from the supply/wet reservoir
(5). When the air pressure in the supply/wet reservoir
(5) of the tractor falls below the warning level, due to a
compressor failure or excessive leakage on the tractor, the
warning devices will start to operate. In the illustration,
the one-way check valve (7) has prevented the reservoir
air pressure in the primary/dry reservoir (8) from escaping
back to the supply/wet reservoir and the ruptured line.
Loss of reservoir air pressure
5
7
8
The truck protection system described is an example of
a tractor equipped with a type of cab-mounted trailer
supply valve (28) which will close automatically when
the air pressure in the supply (emergency) line (21) drops
below 45–20 psi. The valve may also be closed manually.
In this illustration the pressure has been lowered to
approximately 45–20 psi and the tractor protection
system has closed automatically, placing the trailer brakes
into an emergency position. Also, the spring parking
brakes system has had the air pressure released activating
the spring parking brakes.
Pressure lowered to 45–20 psi
28
26
Air Brake Manual
49
Manual trailer supply valve
Trailer spring parking brakes
Some older tractors may be equipped with a different
type of cab-mounted trailer supply valve which must be
operated manually by the driver. It has two positions:
normal and emergency.
Spring parking brakes are now commonly found on
trailers. Spring parking brakes serve as a means of
securing a parked trailer whether it is attached to a tractor
or not. The spring parking brakes are applied with spring
pressure and not air pressure, so there is no risk of the
parking brakes releasing and the trailer moving. They also
act as an emergency braking system if the trailer were to
break away from the tractor or if the tractor lost adequate
air pressure.
The tractor will be equipped with a tractor protection
valve, and the trailer unit with a relay emergency valve,
as in the previous system.
The functions of the trailer supply valve, tractor protection
valve and relay emergency valve will be similar to those
explained previously. However, there is one important
difference. In the event of tractor reservoir air loss, the
trailer supply valve must be shifted to the emergency
position manually to seal off the tractor.
Any time the driver shifts the cab-mounted trailer supply
valve to the emergency position, and the trailer system is
charged, the trailer supply valve will exhaust the supply
(emergency) line, which causes the trailer reservoir to
dump its air directly to the trailer brake chambers.
The trailer brakes will remain applied only as long as air
pressure remains within the trailer system. How long the
air in the system will hold the brakes applied depends on
how airtight the system is. As a safety precaution, parked
trailers without spring parking brakes should always
have the wheels blocked to prevent a possible runaway.
To move a trailer that has been parked with the brakes in
an emergency application, it is necessary to charge the
system to release the trailer brakes.
A trailer with spring parking brakes has these components:
• f ront and rear service reservoirs (16)
• t railer spring brake valve (18)
• r elay valve (17) (the same as on a tractor — not an
emergency relay valve used on trailers without spring
parking brakes)
• spring
parking brake chambers (15) (same as on
a tractor)
The trailer spring brake valve is responsible for several
important functions:
1. I t controls the application and release of the trailer’s
spring brakes.
2. It protects and isolates the front service reservoir
from the rear service reservoir. This is important as it
prevents an automatic application of the trailer spring
brakes if one of the reservoirs were to lose air pressure.
3. It prevents automatic spring brake application if the
trailer supply line has a gradual leak.
4. It will automatically apply the spring parking brakes if
supply pressure is rapidly lost (i.e.: trailer breakaway).
Trailer spring parking brakes
22
18
17
21
15
16
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Air Brake Manual
Section summary questions
1. What is the purpose of a two-way check valve?
2.Why should the glad hands be protected when
not in use?
3.How can a driver control the trailer
brakes independently?
4. Should the hand valve be used for parking?
5. What is the purpose of the tractor protection valve?
6.How can you test the proper function of the tractor
protection valve?
7. What is the purpose of the trailer supply valve?
8. Name three functions of the relay emergency valve.
9.Describe the function of the supply (emergency) line.
10.Describe the function of the control (service) line.
11.What will occur if the supply (emergency) line ruptures?
12.What will occur if the control (service) line ruptures?
13.What will occur if a brake application is made with a
ruptured control (service) line?
14.If the foot valve and the hand valve are operated at the
same time, can the application air pressure be greater
than the reservoir air pressure?
15.Why is a trailer equipped with spring parking brakes?
16.What brake components are on a trailer equipped
with spring parking brakes?
17.What are the four functions of the trailer spring
brake valve?
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51
Notes
52
Air Brake Manual
Section 6
Dual air brake system
6
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53
Dual air brake system
As its name suggests, the dual system is two systems or
circuits in one. There are different ways of separating the
two parts of the system. On a two-axle vehicle, one circuit
operates the rear axle and the other circuit operates the
front axle. If one circuit has a failure, the other circuit is
isolated and will continue to operate.
Most air brake equipped vehicles on the road today
are using a dual air brake system. The system has been
developed to accommodate a mechanically-secured
parking brake that can be applied in the event of service
brake failure. It also accommodates the need for a
modulated braking system should either one of the two
systems fail. It is actually two brake systems in one, with
more reservoir capacity resulting in a much safer system.
At first glance, the dual system might seem complicated,
but if you understand the basic air brake system described
so far, and if the dual system is separated into basic
functions, it becomes quite simple.
Basic dual air brake system
In the illustration, air is pumped by the compressor (1)
to the supply/wet reservoir (5) (blue), which is protected
from over pressurization by a safety valve (4). Pressurized
air moves from the supply/wet reservoir to the primary/
dry reservoir (8) (green) and the secondary/dry reservoir
(10) (red) through one-way check valves (7). At this point,
the dual circuits start. Air from the primary/dry reservoir
is directed to the foot valve (31).
Basic dual air brake system
29
31
1
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Air Brake Manual
4
5
7
8
9
10
Dual-circuit foot valve
Air is also directed from the secondary/dry reservoir to the
foot valve. The foot valve is similar to the one described
earlier in the basic air brake system, but is divided into
two sections (two foot valves in one). One section of this
dual foot valve controls the primary circuit and the other
controls the secondary circuit. When a brake application
is made, air is drawn from the primary reservoir through
the foot valve and is passed on to the rear brake chambers.
At the same time, air is also drawn from the secondary
reservoir, passes through the foot valve and is passed on
to the front brake chambers. If there is air loss in either
circuit, the other will continue to operate independently.
Unless air is lost in both circuits, the vehicle will continue
to have braking ability. The primary and secondary circuits
are equipped with low air pressure warning devices, which
are triggered by the low air pressure indicator switch (9)
and reservoir air pressure gauges (29) located on the dash
of the vehicle.
Treadle
Dual air brake system with
spring parking brakes
Supply
ports
When spring brakes are added to a dual air brake system,
the same type of dash control valve discussed previously
is used. Blended air is used to supply the spring parking
brake control valve (27). Blended air is air taken from the
primary and secondary circuits through a two-way check
valve (26). With this piping arrangement the vehicle can
have a failure in either circuit without the spring brakes
applying automatically. If air is lost in both circuits, the
spring brakes will apply.
Delivery
ports
Exhaust port
Dual air brake system with spring parking brakes
27
26
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55
Spring parking brakes with
modulator valve
Spring parking brakes in this system serve two purposes:
first as a parking brake, and second as an emergency
braking system. If a failure occurs in the primary circuit
(green), and a brake application is made, control air from
the foot valve is directed to a spring brake modulator valve
(23). As there is no supply air to maintain balance in the
modulator valve, because of the primary circuit failure,
the modulator valve then exhausts air pressure from the
spring parking brake circuit. The amount of air released is
equal to the amount of air applied by the foot valve. The
release of air in the spring parking brake circuit causes the
drive axle to brake using spring pressure (12).
When the brakes are released, supply air from the
secondary circuit (red) returns the spring parking brakes
to an off position. Brake applications can be repeated until
all the air from the secondary circuit is lost. However as
the air pressure drops below 85 psi, the spring parking
brakes won’t return to the full off position — in fact they
will start to drag. At approximately 35 psi, the spring
parking brake control valve (27) on the dash will exhaust
the remaining air in the secondary circuit and the spring
parking brakes are fully applied. The only way the vehicle
can be moved after all air is lost is to repair the damaged
circuit and recharge the system, or cage the spring parking
brake system.
Spring parking brakes with modulator valve
27
56
Air Brake Manual
23
12
Combination tractor and trailer
with spring parking brakes
The trailer system is supplied by blended tractor air taken
from the primary and secondary circuits through a twoway check valve as previously described.
The system is charged by opening the trailer supply valve
(28), allowing air from the tractor to pass through the
tractor protection valve (24) and the trailer spring brake
valve (18) directly into the trailer spring parking brake
chambers (15). When air enters, the pressure protection
part of the trailer spring brake valve opens, allowing the
air to fill the trailer reservoirs. The trailer spring brakes
will not release until the reservoir pressure on the trailer
is adequate.
When a brake application is made, blended control air acts
on the relay valve (17), which releases air from the trailer
reservoir to the brake chambers.
In a dual air brake system, if one circuit develops a leak, the
other circuit would be protected from air pressure loss by
the two-way check valve (26).
If the trailer breaks away from the tractor, the control
(service) and supply (emergency) lines will be pulled apart.
The sudden loss of air in the supply (emergency) line will
cause the trailer supply valve to close, which will cause
the tractor protection valve to close, preventing air from
escaping out of either broken connection. The air supply
in the tractor is sealed off and is available to control the
tractor brakes. At the same instant, the sudden loss of air
in the supply (emergency) line causes the trailer spring
parking brake valve to exhaust the air from the trailer
spring parking brake chambers, applying the trailer
brakes. The trailer brakes cannot be released under these
conditions unless the lines are re-coupled and the trailer
reservoirs recharged.
If only the supply (emergency) line breaks between tractor
and trailer, the same sequence of events will occur.
A break or rupture in the control (service) line will not
affect the trailer until a brake application is made. A loss of
pressure in the tractor system will then result if pressure
is allowed to drop low enough it will cause the same
emergency brake application described above. However,
the driver will be able to release the spring parking brakes
by releasing the foot valve, rebuilding air pressure and
opening the trailer supply valve.
To apply the spring parking brakes, the spring parking
brake control valve (27) is closed, causing a loss of air
pressure in the line which applies the spring parking
brakes as described above.
The old and new tractor and trailer systems are fully
interchangeable, whether they are a dual air brake system
or basic air brake system, and whether they are systems
with or without spring parking brakes.
Combination tractor and trailer with spring parking brakes
27
28
26
24
18
12
17
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57
Section summary questions
1. What is the basic principle of the dual air brake system?
2.What valve is used to protect the primary circuit from
the secondary circuit?
3.In a dual air brake system, will the vehicle continue
to have braking ability if one circuit fails?
4.Is there a difference between the foot valve used in a
basic air brake system and the foot valve used in the
dual air brake system?
5.Name two functions of the spring parking brakes in
a dual air brake system.
6.Describe the functions of the spring brake
modulator valve.
7.If the trailer breaks away from the tractor on a dual air
brake system, what applies the brakes on the trailer?
8. What is blended air?
9.Can a trailer with a basic air brake system be towed by
a tractor with a dual air brake system?
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Air Brake Manual
Section 7
7
Electronic controlled
braking and
tracking systems
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59
Electronic controlled
braking and
tracking systems
Anti-lock brake system (ABS)
An anti-lock brake system is an electronic system that
monitors wheel speed at all times and controls wheel
speed while braking. If it detects a wheel locking up during
a brake application, the system releases brake pressure to
that wheel only. This keeps the wheel from skidding and
increases vehicle stability and control during emergency
stops and in adverse situations such as on wet or iceslicked roads, through curves or during lane changes.
The air brake system remains the same with the addition
of the ABS components.
An ABS basically consists of:
• an electronic control unit (ECU)
• a wheel sensor and tooth wheel
The ECU is the brain of the system. The wheel sensors are
located on a minimum of two or up to all six sets of wheels,
depending on the system. The wheel sensors constantly
send information to the ECU. When a brake application
is made and the system detects a wheel locking up, the
ECU sends a message to the ABS valves to release brake
pressure on that particular wheel, preventing lockup.
Usually there is a warning light located on the dash to
inform the driver of the system operation. Every driver
should have full understanding of the warning light
operation for the vehicle they drive.
When driving a vehicle with ABS apply the brakes
normally. When the ABS starts working, do not release
the pressure you have applied to the brake pedal. Avoid
pumping the brake pedal as the ABS automatically applies
and releases the brakes up to five times a second which
is much faster than you can pump the brake pedal. For
optimum ABS operation, the driver should make a brake
application and hold, allowing the ABS to control braking
pressure at each wheel.
• ABS valves
When using an engine brake and you encounter a slippery
road surface, the ABS will detect the wheel lockup and
automatically turn off the engine brake until traction is
regained, then resume engine braking.
Tooth wheel and sensor block
For information specific to the vehicle you drive, refer to
the operator’s manual.
Trailers may also be equipped with ABS and the system
will operate much the same as on the tractor. However,
trailers with ABS will have a system warning light typically
mounted on the trailer’s left front corner, visible in the
left side mirror. On some systems there may be an extra
warning light on the dash of the tractor.
Tractors and trailers with or without ABS can be
connected to one another without affecting the operation
of the air brake system.
Axle
Wheel
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Air Brake Manual
Four sensor-four modulator valve anti-lock brake system
Relay block
Tooth wheel and
sensor block
Modulator
valves
Brake
chambers
Electronic
control unit
(ECU)
Tooth wheel
and sensor
block
Tooth wheel and
sensor block
Dash lamp
Brake chambers
Brake pedal
Brake
chambers
Modulator
valves
Tooth wheel and
sensor block
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61
Automatic traction control (ATC)
Section summary questions
Automatic traction control is an electronic system that
monitors wheel spin on acceleration and uses controlled
braking to gain traction. This system reduces the potential
of powering into a jackknife caused by wheel overspin
during acceleration, along with improving the ability of
the driver to manoeuvre the vehicle on slippery surfaces
through curves and lane changes. ATC is an option only
available on vehicles equipped with ABS.
1. What is the purpose of the anti-lock brake system?
The system uses two functions: differential braking and
electronic engine control.
Differential braking occurs when a drive wheel begins to
spin, causing the ATC to automatically apply the brake on
that wheel and transfer power to the other driving wheels.
Differential braking is activated when the system detects
a wheel slipping because the wheels are operating on
different surfaces such as one wheel on dry pavement and
the other on ice.
Electronic engine control is activated automatically by the
ATC if all the wheels begin to slip. Engine power is reduced
to attain optimum tire-to-road traction without driver
intervention. When a vehicle is being operated with the
cruise control set and wheel slip is detected, the ATC will
automatically cancel the cruise control setting. This will
aid the driver in maintaining control. Cruise control should
not be used on slippery surfaces.
A warning light is located on the dash to inform the driver
when wheel slip is detected. The ATC system uses many of
the same sensing and control components as the anti-lock
brake system. For information specific to the vehicle you
drive, refer to the operator’s manual.
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Air Brake Manual
2.What three components does an anti-lock brake
system consist of?
3.How should you apply the brakes when braking
on wet or icy roads in a vehicle equipped with
anti-lock brakes?
4.Can a trailer with anti-lock brakes be connected
to a tractor without anti-lock brake system?
5.What is the purpose of the automatic traction
control system?
6.What two functions does the system use to
control traction?
Section 8
Brake adjustment and
in-service check
8
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63
Brake adjustment and
in-service check
Brake adjustment
On vehicles equipped with hydraulic brakes it is possible
to pump the brake pedal to compensate for brakes that
are out of adjustment. This is not possible with a vehicle
equipped with an air brake system with manual slack
adjusters. When the brakes are adjusted by slack adjusters,
the shoes are moved toward the drum. This brings them as
close to the drums as possible minimizing the amount of
free travel when the brakes are applied. It also reduces the
volume of air used to apply the brakes.
Brake adjustment (pushrod travel) must be checked as
part of the pre-trip air brake inspection (Section 9).
Your company may have a maintenance crew to keep
vehicles safely running. But one person alone is ultimately
responsible to ensure that the brakes are operating
properly before the vehicle is moved. That person is
the driver.
There is no
reliable substitute
for physically
checking brake
adjustment.
Definitions
Slack adjustment means adjusting the brakes to keep air
chamber pushrod travel within tolerance to ensure that
the brakes are effective.
Brake adjustment — it’s critical
Pushrod travel means the distance the pushrod extends
from the air brake chamber when the brakes are applied.
The most important part of any braking system is the
foundation brakes. If these are not in correct working
order and properly adjusted, the best designed air
brake system won’t be able to safely stop the vehicle.
S-cam brake — released
Unfortunately, brake adjustment is often neglected.
The reality is that brake failures and runaways that result
in crashes are almost never caused by an air
system failure, but by the absence of routine brake
maintenance, or by the driver failing to check brakes
on a daily basis.
Daily pre-trip inspections are mandatory. The driver may
be held responsible if the brakes are incorrectly adjusted or
not working properly.
Brake adjustment — it’s the law
Federal and provincial laws require that manual and
automatic slack adjusters be checked daily, during the
pre-trip inspection. It is up to you, the professional driver,
to ensure your vehicle has safe, correctly-adjusted brakes.
You the driver, can be fined if the brakes on the vehicle
you are driving are found to be out of adjustment.
You’re also required by law to check your brakes
before driving down steep grades that are posted with
regulatory signs.
You can’t always crawl underneath your vehicle to
measure air brake chamber pushrod travel, but there
are devices available to help you visually check for
brake adjustment.
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Air Brake Manual
The most common type of air brake chamber used on
trucks and trailers is the Type 30. These chambers have
a maximum available stroke of 2M inches. A long stroke
Type 30 is designated Type 30LS, and has a maximum
available stroke of three inches.
S-cam brake in the released position
S-cam brake — applied
In the illustration below, the brakes have been applied
and the brake linings have contacted the brake drum. The
brake chamber has stroked less than M of its maximum
stroke, indicating that the brake is in correct adjustment.
Each brake on an axle should be adjusted to have a similar
amount of pushrod travel — that is, the pushrod travel of
the left brake should be similar to the pushrod travel of the
right brake on the axle.
S-cam brake in the applied position
S-cam brake — incorrectly
adjusted and cold brake drum
Note: Check air brake chamber pushrod travel at least daily.
In the illustration below, the brakes are applied and the
linings have contacted the brake drum.
This brake chamber pushrod has excessive travel, but may
work as long as the brake drum is cool. But it won’t stay
cool for long!
Note that the brake chamber has stroked more than M of
its maximum, which means that this air brake chamber
pushrod has excessive travel and the brakes are in need
of adjustment.
When the brake drum is cool and with normal brake
application pressure, the brake will seem to be effective,
so it is easy to be lulled into a false sense of security.
Air brake-equipped vehicles rarely “lose” their brakes.
Rather, brake failure occurs because the driver has failed
to check and correctly adjust the brakes.
S-cam brake incorrectly adjusted and
cold brake drum
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65
S-cam brake — incorrectly
adjusted and hot brake drum
Checking and adjusting S-cam
brakes — manual slack adjusters
Now that the brake drum is hot, the brake linings will
no longer contact the drum. The brake chamber has
bottomed out.
Drivers seldom have the luxury of having the use of a pit
and hydraulic jacks so that brake adjustment can be done
with wheels off the ground. However, brakes can still be
adjusted very accurately with the wheels on the ground
using one of the following methods.
The illustration below shows the same brake after a few
brake applications.
Cast iron brake drums expand when heated, causing
the air chamber to stroke further and further as the
temperature rises.
If an unforeseen event required the driver to make a
sudden stop, the brake chamber could bottom out and
braking power would be greatly reduced.
On long downgrades, the expansion of hot brake drums
can cause a total loss of braking and result in a runaway.
Be sure that the vehicle is safely parked with the wheels
blocked. The air system should be at full pressure and the
spring parking brakes released. Turn the engine off so that
you can listen for air leaks.
There are two methods of checking for correct
adjustment, but the measurements that indicate the need
for adjustment are different.
Pry method of free stroke measurement
Even incorrectly adjusted brakes may seem to work when
brakes are lightly applied.
The illustration below shows the common method of
checking the free stroke of the pushrod — using a brake
tool to measure pushrod free stroke.
This could result in brake fade under moderate to heavy
braking, or on a downhill.
Free stroke measurement
S-cam brake incorrectly adjusted and
hot brake drum
Make sure you have the proper equipment to check and
adjust the brakes. You will need:
• a wrench
• a marker to measure pushrod travel
• sturdy clothing suitable for crawling under the vehicle
• a flashlight
You should also wear eye protection and a bump cap.
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Air Brake Manual
To begin, make sure system air pressure is at least
90–100 psi and all parking brakes are released.
While checking air brakes look for other brake component
wear and excessive play including:
1. M
ake a mark on the pushrod where it exits the
brake chamber.
• S-cam bushing wear
Note: It is sometimes difficult to mark the pushrod at this
location. Alternatively, mark the centre of the clevis pin on
the slack adjuster arm.
2. Pull the pushrod out from the brake chamber, using a
tool for leverage.
3. Measure the distance between the chalk mark and the
face of the brake chamber M to P of an inch of free stroke
is a good range for a type 30 standard stroke brake
chamber. If this distance is more than P of an inch, the
brakes need adjustment.
Applied stroke method
(service brake application)
The applied stroke (brake application) method requires
two people — one to apply the brakes and one to
measure travel.
If you have a device to apply and hold the service brakes
on, you can use the brake application method without
needing another person.
• S-cam spline wear
• Slack adjuster play (worn gear)
• Clevis pin wear
Brake adjustment indicators
New air brake chamber pushrods have a marking (usually
red) to indicate when brake adjustment must be done
immediately. If the pushrod travel becomes excessive, the
marking will show.
Don’t wait until the red marking is exposed before
adjusting the brakes.
Note: If the red marking on the pushrod (at the chamber face) is
visible, it indicates that the brake is dangerously out of adjustment
and it needs immediate attention.
Pushrod with indicator marking
Applied stroke measurement
The applied stroke method is the preferred method and
is also a method recommended by commercial fleet
maintenance supervisors.
1. W
ith the brakes released, make a mark where the
pushrod exits the brake chamber.
2. With the engine off, make a series of brake applications
to reduce the reservoir pressure to between 90 to
100 psi.
3. Apply and hold a full brake application (90 to 100 psi).
4. Measure the distance between the mark and the
face of the brake chamber. The difference between
measurements is called the chamber applied stroke.
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67
Brake adjustment — manual
slack adjuster
Adjustment technique
With a typical Type 30 standard stroke brake chamber,
you must adjust the brakes if pushrod travel is:
• more than P of an inch using the pry method of free
stroke measurement
• more than 11/@ inches using the brake application —
applied stroke measurement method
Note: These are maximum measurements. You should adjust the
brakes if your measurements approach these limits.
Optimal pushrod travel
Stroke with
brake applied
Correct adjustment
Most manual slack adjusters have a spring-loaded locking
sleeve that must be pressed in and held so the adjusting
bolt can be turned.
Depending upon the orientation of the slack adjuster,
the correct direction to turn the adjusting bolt may be
clockwise or counterclockwise. Watch to ensure that you
are tightening and not loosening the slack adjustment.
With a wrench of the proper size, usually a 9/!^ inch,
depress the locking sleeve and turn the adjusting bolt
while watching the end of the camshaft. The camshaft
will rotate slightly as the bolt is being turned. If you are
turning in the right direction, the cam will rotate in the
same direction that it would if the brakes were being
applied, as shown by the arrow on the cam end in the
illustration below.
Note: The pushrod and the slack adjuster arm should never move
away from the brake chamber while turning the adjusting bolt.
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Air Brake Manual
Incorrect adjustment
If when turning the adjusting bolt, the slack adjuster
is pulling the pushrod out of the chamber, stop. The
adjusting bolt is being turned in the wrong direction.
Once the proper direction is established, continue turning
until solid resistance is met. This indicates that the brake
linings have contacted the brake drum.
If the brake has no dust shields, or if you can see the brake
shoes and linings through an inspection slot, you can
visually verify that the linings have contacted the drum.
Backing off the adjusting bolt about 1/# of a turn should
establish correct running clearance between the lining
and drum. Be sure the locking sleeve re-engages the bolt
so that the adjustment will not back off.
It is common, especially on tandem axle units, for the
adjusting bolts on one axle to adjust in one direction,
while the other axle requires an opposite turn. It is not
uncommon to find that the brakes on one or the other
axle have been mistakenly backed off, creating a serious
safety hazard.
After adjusting, verify that there is sufficient clearance
by again pulling or prying on the slack adjuster arm,
and re-measuring the free stroke, remember that M inch
is ideal.
Adjusting S-cam brakes
with automatic slack
adjusters — why, when and how
Automatic slack adjusters are entirely different than the
manual types.
Once properly installed, automatic slack adjusters should
not need manual adjustment. If an automatic slack
adjuster is found to stroke beyond the maximum allowed,
this usually indicates that there are brake problems that
need to be repaired by qualified brake service personnel.
Manufacturers recommend that automatic slack adjusters
be checked by a mechanic at each chassis lubrication
interval, or from 40,000 to 80,000 km. However, the
National Safety Code of Canada, as well as federal and
state laws in the United States require a daily check of
brake adjustment as part of a pre-trip inspection. Also,
in certain mountainous areas of North America, trucks
are required to stop and check brakes before proceeding
down long grades. The checking is required regardless of
whether manual or automatic slack adjusters are used.
Prior to manually checking slack adjuster stroke, ensure
tractor spring parking brake control valve and trailer
supply valve are open. Ensure reservoir air pressure is
above 90 psi and make a full brake application and
release. Repeat this process six times — these brake
applications will be adequate to cause an automatic slack
adjuster to adjust. A light brake application will not cause
the adjustment.
If after making six full brake applications with reservoir
pressure above 90 psi, a pushrod stroke is excessive, the
automatic slack adjuster has either failed, been incorrectly
installed, or there is a problem within the foundation
brake. Manual adjustment at this time is only a stop-gap
measure so the vehicle can be driven to a service depot for
inspection and possible replacement.
Most manufacturers of automatic slack adjusters specify
that pushrod stroke be checked by making a 90 to 100 psi
application. If you have no application pressure gauge,
turn the engine off and pump the reservoir pressure
down to between 90 and 100 psi and then make a
full application.
Automatic slack adjusters should only be adjusted or
repaired by a qualified mechanic except in an emergency.
Follow the manufacturer’s instructions in carrying out
any adjustment.
Automatic slack adjusters still need to be regularly
checked to ensure that correct adjustment is being
maintained.
Pushrod strokes with automatic slack adjusters are usually
slightly longer than with well-adjusted manual slack
adjusters. While a manual adjustment may temporarily
shorten the stroke, the automatic slack will soon re-set to
its designed stroke.
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69
Emergency manual brake
adjustment — automatic
slack adjusters
If the vehicle must be driven to a service depot, an
emergency manual adjustment may be done.
These are emergency procedures only. A repair or
replacement must be made as soon as possible.
Be sure that the vehicle is safely parked with the wheels
blocked. The air system should be at full pressure and the
spring parking brakes released.
With a typical Type 30 standard stroke brake chamber, you
must adjust the brakes if pushrod measurements are:
• more than P of an inch using the pry method of free
stroke measurement
• more than two inches using the brake application — applied stroke method
Note: These are maximum measurements. You should adjust the
brakes if your measurements approach these limits.
Four types of automatic slack adjusters
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Air Brake Manual
If you make an emergency adjustment of an automatic
slack adjuster, be sure to record it on your daily post-trip
inspection report.
It is strongly recommended that you obtain the service
data books for the make of automatic slack adjusters
you are using. They contain all the information needed
to keep them operating properly, including lubrication
requirements, initial setup, inspection and testing. They
are available where truck parts are sold.
Slack adjusters with hexagonal adjusting bolts
Slack adjusters with square adjusting bolts
If the slack adjuster has a hexagonal (six-sided) adjusting
bolt, the brakes may be set up by turning the adjusting bolt
in a clockwise direction until the lining contacts the drum.
Backing off the adjusting bolt by half a turn should restore
running clearance. Backing off may take considerable force
and may be accompanied by a ratcheting sound and feel.
This is normal.
If the slack adjuster has a square adjusting bolt located at
the bottom end of the body, do not attempt adjusting until
a spring-loaded pawl that meshes with internal teeth is
disengaged. These units have a P inch hexagonal (hex) cap
located on the slack adjuster body. Some of these hex caps
are equipped with a round “button” that can be pried up
approximately 1/#@ of an inch and held, using a screwdriver.
On units not equipped with the button, the hex cap, spring
and pawl must be removed. With the spring and pawl
disengaged, an adjustment can be made.
These brakes must be set up with a counter-clockwise turn
until the lining contacts the drum. Turning the adjusting
bolt half a turn clockwise will restore running clearance.
Release the button or re-install the spring and pawl if they
were removed.
Adjusting
bolt
To “set up”
Button
To “set up”
Adjusting
bolt
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71
Disc brake adjustment
In-service checks
There are a number of makes and models of air disc brakes,
each with a different adjustment procedure. It is therefore
recommended that you consult the manufacturer’s
manual for adjustment or service.
In some areas signs are posted in advance of steep or long
downgrades. These signs indicate that the driver must
stop the vehicle in the pullout area and inspect the braking
system before proceeding. Check that:
Wedge brake adjustment
• The compressor is maintaining full reservoir air pressure.
Manual adjusters:
• Hoist or jack wheels off the ground.
• Remove dust cover from adjusting slots at two places on
each brake. On twin chamber units the adjusting slots
are in the backing plate below the forward chamber and
above the rear chamber. If star wheel adjusting bolts
are not found at these positions, the brake has been
assembled on the wrong side of the vehicle. The single
chamber units have adjusting slots in the backing plate
on each side of the chamber.
• Pushrod travel is within limitations on all chambers.
• There are no air leaks.
• The glad hands and the lines are secure.
• The drums, bearings and tires are not overheating.
• The trailer supply valve is operating properly.
This should be used as an in-service check only and not
mistaken as a daily pre-trip air brake inspection.
The driver must be aware of the condition of the vehicle’s
braking system at all times. This can be done by:
• Adjusting bolts have right-hand threads. Turning the
wheel by hand, use an adjusting spoon to turn the star
wheel until the shoe develops a heavy drag on the drum.
Then back off the bolt to a very light drag on the drum.
• watching the pressure gauges
• Repeat for the other shoe on that brake assembly.
By this method of observation, the driver should be able to
notice any defects developing in the braking system and
be aware that service or adjustment is required.
• Replace the dust covers in the adjusting slots.
• Repeat the above steps on the other brake assemblies.
• hearing the warning signals
• feeling the braking response of the vehicle
After a brake adjustment
At the first stop after adjusting the brakes, check each
brake drum or rotor for temperature. An extra hot brake
drum indicates that you have over adjusted it.
Only when all brakes are properly adjusted will the system
be able to absorb the required amount of energy to bring
the loaded vehicle to a safe stop under extreme conditions.
Remember that poorly adjusted brakes may appear to be
working at slow speeds on light air application. The only
way to be sure your vehicle brakes are properly adjusted is
to physically check the pushrod travel at each wheel.
Maintenance and servicing
the air brake system
Ensuring that the vehicle is in proper condition to drive
is the responsibility of everyone involved.
Preventive maintenance checks are designed to prevent
mechanical failures that could result from neglect.
These checks cut repair costs by detecting minor
mechanical defects which would otherwise develop
into major breakdowns.
Preventive maintenance may be assigned to a
maintenance crew or to the driver. However, the driver
must ensure that the braking system is in safe operating
condition before the vehicle moves.
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Air Brake Manual
Air brake adjustment myths
Section summary questions
There is some misinformation about air brakes that you
may hear. Most of this is not critical, but the following
three myths could be dangerous, if believed.
1. Why is it so critical to check slack adjustment?
Myth #1: Brake adjustment can be checked from the cab by
making a full brake application and checking for an initial pressure
drop of between 8–12 psi. The assumption is that as the brake
chambers stroke further and further, more air volume will be
required, and this should show up on the reservoir gauges.
Fact #1: Modern trucks have very large air reservoirs,
and even if all the brakes had excess pushrod travel, the
pressure drop would not reach the 8–12 psi range. Also,
most truck reservoir gauges do not have markings fine
enough to accurately estimate such pressure changes.
Myth #2: With the brakes applied, a 90-degree angle between
the centre of the slack adjuster arm and the chamber pushrod is a
good indication that the brake is in correct adjustment.
Fact #2: The 90-degree angle is more dependent on the
length of the chamber pushrod than on brake adjustment.
Also, to prevent interference between the slack adjuster
and suspension parts, some manufacturers will vary the
angle up to plus or minus 10 degrees.
2.Who is ultimately responsible for the brakes
on a vehicle?
3.Who will be fined for operating a vehicle with brakes
improperly adjusted?
4.What are the dangers of operating a vehicle where the
pushrod travel is barely within tolerance when the
brake drums are cold?
5.What items should you carry to measure and adjust
slack adjusters?
6.What is the first thing you should do when preparing to
measure slack adjustment?
7.How much pressure should you apply to the foot valve
when measuring slack adjustment using the applied
stroke method?
8.How do you know you are turning the adjusting bolt in
the correct direction when adjusting a slack adjuster?
9. What is an advantage of automatic slack adjusters?
Myth #3: A clockwise turn of the adjusting bolt on a manual
slack adjuster will set up the brakes.
Fact #3: Depending on the orientation of the slack
adjuster on the brake assembly, the correct direction to
set up the brakes may be clockwise, or counter-clockwise.
On vehicles with tandem axles, it is common to find that
one axle sets up with a clockwise turn, while the other
requires a reverse direction.
Air Brake Manual
73
Notes
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Air Brake Manual
Section 9
Pre-trip air
brake inspection
9
Air Brake Manual
75
Pre-trip air
brake inspection
Vehicle inspection
Air brake practical test
An air brake inspection must be carried out on every
air brake-equipped vehicle before you start driving it for
the day.
When upgrading your driver’s licence from single to
combination vehicles with air brakes, you will be required
to satisfactorily demonstrate the complete pre-trip air
brake inspection for combination vehicles.
Every driver of a commercial motor vehicle must carry out
an inspection and log it as outlined in The Highway Traffic
Act and its Regulations. A commercial motor vehicle is one
of the following:
• a motor vehicle with a GVWR of 4,500 kg or more that is
a public service vehicle or commercial truck
• a bus with a seating capacity of 11 or more, including
the driver, that is used for transportation of persons or
property on a highway for gain or compensation
• a school bus
The inspection will take place before the vehicle’s first trip
of the day, and if a trip lasts more than one day, before the
trip begins on the first day, and no later than the first rest
stop on the second and every subsequent day.
Regardless of the maintenance policies of companies of
vehicle owners, all drivers must be capable of determining
if their vehicle is in safe operating condition as outlined in
The Highway Traffic Act and its Regulations.
A person who carries out a vehicle pre-trip inspection
must make a trip inspection report in legible writing that
contains the following:
• the licence plate number or unit number of the vehicle
and any trailer towed by it
• the date of inspection
• a list of any safety defects
• if no items are defective or required to be reported on
the vehicle or any trailer towed by it, a statement to
that effect
• the name and signature of the person who carried out
the inspection
If a driver believes or suspects that there is a safety defect
they must report that belief or suspicion to the carrier.
If the defect poses an unreasonable risk it must be
reported immediately. Any other case must be reported
in a timely manner.
Every commercial vehicle driver must have a current trip
inspection report in their possession for the vehicle.
Checking a vehicle before starting out, as required by
law, will often prevent costly delays as well as reduce the
chances of an accident resulting from a mechanical failure.
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Air Brake Manual
The pre-trip air brake inspection for combination units
should be completed within 30 minutes and within
20 minutes for single units. Failure to complete the
test within the above time limit may result in the test
being discontinued.
The air reservoirs must be drained or fanned down to zero
pressure prior to the pre-trip inspection to simulate that
they were drained during the post‑trip inspection.
You are required to bring all material/tools to complete
the air brake test (i.e. wrench, chalk, watch, etc.). It is
recommended that you wear safety glasses and a bump
cap when performing a pre-trip inspection.
Practical examination will be conducted in either the
metric or imperial systems of measurement, whichever
coincides with the equipment being operated.
Pre-trip inspection — combination unit
The air brake pre-trip inspection when performed
correctly will identify any malfunctioning components
or problems with the system. Failure to complete
a thorough pre-trip inspection of the air brake system
could result in a vehicle being operated on the highway
with a faulty brake system, which could potentially
be deadly. Completing a thorough pre-trip inspection
will allow the driver to operate the vehicle with reduced
chance of break down or system malfunction. It is
the driver’s responsibility to ensure the air brake
system is safe and functioning properly before moving
the vehicle.
Vehicle Secure
A.1Ensure parking brake is applied (trailer supply
valve and tractor spring parking brake control
valve are closed), wheels are blocked and parked
on level ground.
A.2Check reservoir air pressure is at 0 psi and the
compressor is secure, no oil or coolant leaks and
drive belts for wear and tension.
A.3Visually inspect all brake system air hoses, glad
hands, chambers, pushrods, slack adjusters,
drums for damage and are mounted properly
and secure.
Compressor and warning devices check
A.4Start the engine and check that the gauges
are functioning properly and low air pressure
warning devices are operating.
A.5Build air pressure at high idle, ensure reservoir
air pressure builds from 50 to 90 psi within
three minutes.
A.6Low air pressure warning devices must operate
to a minimum of 60 psi.
A.8Continue building air pressure and check that the
governor places the compressor in the unloading
stage between 115 psi and 135 psi (the air
pressure gauges will stop rising).
A.9Fan the service brake pedal until the air pressure
drops to 90 psi and no lower than 80 psi. Stop
fanning to see if the governor has placed the
compressor in the loading stage (the reading on
the air pressure gauge should increase).
A.10Continue fanning the service brake pedal. Low
air pressure warning devices must operate at a
minimum of 60 psi and the trailer supply valve
must close at a minimum of 20 psi.
Note: Tractor spring parking brake control may or may not
also close.
Tractor and trailer emergency systems check
A.11Ensure the trailer supply valve and tractor spring
parking brake control valve are closed, applying
the spring parking brakes. Exit the cab and
disconnect the supply (emergency) line glad
hands. No air should escape from either of the
glad hands.
A.12Enter the cab and ensure the reservoir air
pressure is between 115 psi and 135 psi, shut
off the engine. Open the trailer supply valve. Air
loss from disconnected glad hand will either stop
immediately or bleed down to no lower than 20
psi when the trailer supply valve closes and the
air loss stops.
A.13Ensure trailer supply valve and tractor spring
parking brake control valve are closed, applying
the spring parking brakes. Start engine to rebuild
to full reservoir air pressure between 115 psi and
135 psi. Exit the cab and re-connect the supply
(emergency) line glad hands.
A.7At 90 psi or above, open the tractor spring
parking brake control valve and the trailer supply
valve, releasing the spring parking brakes.
Apply and release the trailer service brakes with
the trailer hand valve while listening for air to
exhaust at the trailer. This will indicate if the
brakes are releasing and that the glad hands are
not crossed.
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77
Tractor protection valve check
A.14Enter the cab and at or above 90 psi, open the
trailer supply valve. Exit the cab and disconnect
the control (service) line glad hands. No air should
escape from either of the glad hands.
A.15Enter the cab, ensure full reservoir air pressure
between 115 psi and 135 psi, open the tractor
spring parking brake control valve. Shut off
the engine.
A.16Make a full service brake application and hold it.
Air should escape from the control (service) line
to no lower than 20 psi, the tractor protection
valve will close and the air loss will stop. Release
the service brake application.
A.17Close the tractor spring parking brake control
valve and ensure the trailer supply valve is closed;
if not, close it manually. Start the engine and
rebuild air pressure. At or above 90 psi, make
a trailer service brake application with trailer
hand valve. No air should escape from the
disconnected control (service) line. Release the
hand valve application.
A.18Exit the cab and re-connect the control (service)
line glad hands.
A.19Enter the cab and at or above 90 psi, open the
tractor spring parking brake control valve and
trailer supply valve, continue building air pressure
to between 115 psi and 135 psi. Once at full
pressure, shut off the engine.
Air leak check
A.20Make a full service brake application with the
foot valve and hold it for one minute. Air pressure
must not drop more than 4 psi.
A.21Listen for audible air leaks. Release the service
brake application, start the engine and rebuild
to full system air pressure between 115 psi and
135 psi.
Brake adjustment — combination unit
Note: For checking if your brakes are in adjustment you will require
a second person to apply and hold the service brakes on or a device
to apply and hold the service brakes on.
A.22Ensure the tractor spring parking brake
control valve and trailer supply valve are open.
(For vehicles equipped with automatic slack
adjusters, make a full service brake application
and release. Repeat this process six times, ensure
reservoir air pressure is above 90 psi for each full
brake application).
A.23Ensure reservoir air pressure is above 90 psi, shut
off the engine. (For added security, place manual
transmission in low gear.)
Brake adjustment verification for automatic or
manual slack adjusters
For safety reasons inspect the tractor or trailer brakes
separately. This will allow the spring parking brake to
be applied on the brakes that are not being inspected,
preventing the vehicle from moving. Measurements
for slack adjustment are for Type 30 standard stroke
brake chambers.
A.24Exit the cab and make a chalk mark on the
pushrod where it enters the brake chamber.
Repeat for all chambers.
A.25Enter the cab and ensure reservoir pressure is
between 90 psi and 100 psi. With a brake pedal
assisting device or a second person, make a 90 psi
to 100 psi service brake application and hold.
A.26Return to the brake chambers and measure
the pushrod travel by checking the distance
from the chamber to the chalk mark on the
pushrod. Ensure the pushrod travel is correct
for the size and type of brake chamber based
on manufacturer’s specifications. Also, brake
chambers which share the same axle must have
a pushrod travel measurement within 1/$ inch of
each other.
Note: For automatic slack adjusters, the slack adjuster must be
replaced if measurements are not within the above specifications.
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Air Brake Manual
Adjustment for manual slack adjusters only
Brake response tests
For drivers with S air brake endorsement
A.27Ensure the tractor and trailer spring parking
brakes are applied and remove the wheel blocks.
M.1Enter the cab and remove the brake pedal
assisting device releasing the service brakes.
Ensure reservoir pressure is at or above 90 psi.
M.2Return to the slack adjuster requiring adjustment
and locate the adjustment bolt. With the proper
wrench, disengage the adjustment bolt locking
mechanism. Turn the adjustment bolt until the
brake lining comes into contact with the brake
drum. If this can not be confirmed by visually
seeing the lining and brake drum due to the dust
cover, observe the direction of the S-cam shaft
while turning the adjustment bolt. The S-cam
shaft should turn in the same direction as it
would when a brake application is being made.
M.3After tightening the adjustment bolt, turn the
adjustment bolt in the opposite direction 1/$ to M
a turn. Remove the wrench and ensure the
locking device returns to the “locked” position.
Failure to engage the locking device could result
in the adjustment bolt turning and causing the
brake to become out of adjustment with the
application of the brakes. Complete these
steps for all slack adjusters.
M.4Enter the cab; ensure reservoir air pressure is
above 90 psi and make a 90 psi to 100 psi service
brake application and hold. Use a brake pedal
assisting device or a second person to hold
the application.
M.5Return to the brake chambers and measure
the pushrod travel to ensure travel is within
1M inches or within manufacturer’s
specifications. Also brakes which share the
same axle must have a pushrod travel
measurement within 1/$ inch of each other.
A.28Enter the cab and start the engine. Open the
tractor spring parking brake control valve and
place the vehicle in a low forward gear. Gently tug
against the applied trailer spring parking brakes.
They must hold the vehicle.
A.29Close the tractor spring parking brake control
valve and open the trailer supply valve. Gently tug
against the applied tractor spring parking brakes.
They must hold the vehicle.
A.30Open the tractor spring parking brake control
valve and apply the trailer service brakes using
the trailer hand valve. Gently tug against the
trailer service brakes. They must hold the vehicle.
A.31Release the trailer service brake application,
move the vehicle ahead slowly and make a service
brake application with the foot valve to check
brake response.
Suggested driving practices
At the end of the final trip of the day inspect your vehicle
for any damage or problems that may have developed on
the road. Complete a trip inspection report and note any
defects found during this inspection.
This could include the following:
•P
ark the vehicle on level ground, apply the spring parking
brakes,shut off the engine and leave it in gear.
•V
isually inspect all brake system air hoses, chambers,
pushrods, slack adjusters, drums for damage and are
mounted properly and securely.
•O
pen the drain valves on all air reservoirs and allow all air
pressure to escape then close the drain valves.
Note: If one slack adjuster requires adjustment then all slack
adjusters must be adjusted.
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79
Pre-trip inspection — single unit
(Not for air-over-hydraulic brake systems)
The air brake pre-trip inspection when performed
correctly will identify any malfunctioning components or
problems with the system. Failure to complete a thorough
pre-trip inspection of the air brake system could result in a
vehicle being operated on the highway with a faulty brake
system, which could potentially be deadly. Completing
a thorough pre-trip inspection will allow the driver to
operate the vehicle with reduced chance of break down
or system malfunction. It is the driver’s responsibility
to ensure the air brake system is safe and functioning
properly before moving the vehicle.
Vehicle Secure
B.1Ensure the spring parking brake control valve is
closed applying the spring parking brakes, wheels
are blocked and parked on level ground.
B.2Check reservoir air pressure is at 0 psi and the
compressor is secure, no oil or coolant leaks and
drive belts for wear and tension.
B.3Visually inspect all brake system air hoses,
chambers, pushrods, slack adjusters, drums for
damage and are mounted properly and securely.
Compressor and Warning Devices Check
B.4Start the engine and observe the gauges are
functioning properly and low air pressure
warning devices are operating.
B.5Build air pressure at high idle, ensure reservoir
air pressure builds from 50 psi to 90 psi within
three minutes.
B.6Low air pressure warning devices must operate to
a minimum of 60 psi.
B.7Continue building air pressure and check that the
governor places the compressor in the unloading
stage between 115 psi and 135 psi (the air
pressure gauges will stop rising.) Open the spring
parking brake control valve, releasing the spring
parking brakes.
B.8Fan the service brake pedal until the air pressure
drops to 90 psi and no lower than 80 psi. Stop
fanning to see if the governor has placed the
compressor in the loading stage (the reading on
the air pressure gauge should increase.)
B.9Continue fanning the service brake pedal. Low
air pressure warning devices must operate at a
minimum of 60 psi. Build reservoir air pressure at
fast idle to full air pressure between 115 psi and
135 psi.
B.10Ensure the spring parking brakes are released by
opening the spring parking brake control valve
and then shut off the engine.
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Air Brake Manual
Air Leak Check
B.11Make a full service brake application with the
foot valve and hold it for one minute. Air pressure
must not drop more than 3 psi.
B.12Listen for audible air leaks. Release the service
brake application, start the engine and rebuild
to full system air pressure between 115 psi and
135 psi.
Brake adjustment — single unit
(Buses with low ground clearance must be
adjusted by a qualified mechanic)
For testing purposes, applicants operating buses with low
ground clearance will not be allowed to manually adjust
the brakes on the bus.
Note: For checking if your brakes are in adjustment you will require
a second person to apply and hold the service brakes on or a device
to apply and hold the service brakes on. Measurements for slack
adjustment are for Type 30 standard stroke brake chambers.
B.13Ensure the spring parking brake control valve
is open. (For vehicles equipped with automatic
slack adjusters, make a full brake application and
release. Repeat this process six times, ensure
reservoir air pressure is above 90 psi for each full
brake application).
B.14Ensure reservoir air pressure is above 90 psi and
shut off the engine. (For added security, place
manual transmission in low gear.)
Brake adjustment verification for automatic or
manual slack adjusters
B.15Exit the cab and make a chalk mark on the
pushrod where it enters the brake chamber.
Repeat for all chambers.
B.16Enter the cab and ensure reservoir pressure is
between 90 psi and 100 psi. With a brake pedal
assisting device or a second person, make a 90 to
100 psi service brake application and hold.
B.17Return to the brake chambers and measure the
pushrod travel by checking the distance from
the chamber to the chalk mark on the pushrod.
Ensure the pushrod travel is within two inches
for automatic slack adjusters, and within 1M
inches for manual slack adjusters or within
manufacturer’s specifications. Also, brake
chambers which share the same axle must have
a pushrod travel measurement within 1/$ inch of
each other.
Note: For automatic slack adjusters, the slack adjuster must be
replaced if measurements are not within the above specifications.
Adjustment for manual slack adjusters only
For drivers with S brake endorsement
M.1Enter the cab and remove the brake pedal
assisting device releasing the service brakes.
Ensure reservoir pressure is at or above 90 psi.
M.2Return to the slack adjusters requiring
adjustment and locate the adjustment bolt. With
the proper wrench, disengage the adjustment
bolt locking mechanism. Turn the adjustment
bolt until the brake lining comes into contact
with the brake drum. If this can not be confirmed
by visually seeing the lining and brake drum
due to the dust cover, observe the direction of
the S-cam shaft while turning the adjustment
bolt. The S-cam shaft should turn in the same
direction as it would when a brake application is
being made.
Suggested driving practices
At the end of the final trip of the day inspect your
vehicle for any damage or problems that may have
developed on the road and note any defects found during
this inspection.
This could include the following:
•P
ark the vehicle on level ground, apply the spring parking
brakes, shut off the engine and leave it in gear.
• Visually
inspect all brake system air hoses, chambers,
pushrods, slack adjusters, drums for damage and are
mounted properly and securely.
•O
pen the drain valves on all air reservoirs and allow all
air pressure to escape then close the drain valves.
M.3After tightening the adjustment bolt, turn the
adjustment bolt in the opposite direction 1/$ to M
a turn. Remove the wrench and ensure the
locking device returns to the “locked” position.
Failure to engage the locking device could result
in the adjustment bolt turning and causing the
brake to become out of adjustment with the
application of the brakes. Complete these steps
for all slack adjusters.
M.4Enter the cab; ensure reservoir air pressure is
above 90 psi and make a 90 psi to 100 psi service
brake application and hold. Use a brake pedal
assisting device or a second person to hold
the application.
M.5Return to the brake chambers and measure
the pushrod travel to ensure travel is within
1M inches or within manufacturer’s
specifications. Also, brakes which share the
same axle must have a pushrod travel
measurement within N inch of each other.
Note: If one slack adjuster requires adjustment then all slack
adjusters must be adjusted.
Brake response test
B.18Ensure the spring parking brakes are applied and
remove the wheel blocks.
B.19Enter the cab and start the engine. With the
spring parking brake control valve closed. Place
the vehicle in a low forward gear. Gently tug
against the spring parking brakes. They must
hold the vehicle.
B.20Open the spring parking brake control valve,
move the vehicle ahead slowly and make a service
brake application with the foot valve to check
brake response.
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81
Pre-trip inspection — air-over-hydraulic (air actuated)
brake system
The air-over-hydraulic pre-trip inspection when performed
correctly will identify any malfunctioning components or
problems with the system. Failure to complete a thorough
pre-trip inspection of the air brake system could result in a
vehicle being operated on the highway with a faulty brake
system, which could potentially be deadly. Completing
a thorough pre-trip inspection will allow the driver to
operate the vehicle with reduced chance of break down
or system malfunction. It is the driver’s responsibility to
ensure the brake system is safe and functioning properly
before moving the vehicle.
Vehicle secure
D.1Ensure the parking brake is applied, the wheels
are blocked and parked on level ground.
D.2Check reservoir pressure is at 0 psi (0 kPa), the
compressor is secure and it has no oil or coolant
leaks and check the compressor drive belts for
wear and tension. Check the hydraulic fluid
reservoir level and that there is no hydraulic
fluid leaks.
D.3Visually inspect all wheel cylinders and lines for
hydraulic fluid leaks, drums for damage.
Compressor and warning devices check
Air leak check
D.11Make a full service brake application with the
foot valve and hold it for one minute. Air pressure
must not drop more that 3 psi (21 kPa).
D.12Listen for audible air leaks. Release the service
brake application, start the engine and rebuild to
full system pressure between 115 psi (793 kPa)
and 135 psi (931 kPa).
Brake response test
D.13Ensure the parking brake is applied and remove
the wheel blocks.
D.14Apply the parking brake and place the vehicle in
a low forward gear and gently tug against the
parking brake. It must hold the vehicle.
D.15Release the parking brake, move the vehicle
ahead slowly and make a service brake
application with the foot valve to check
brake response.
Suggested driving practices
At the end of the final trip of the day inspect your vehicle
for any damage or problems that may have developed
on the road and note any defects found during this
inspection. The post-trip inspection can be added to the
report you completed during the pre-trip inspection at the
start of that day.
D.4Start the engine and check that the gauges
are functioning properly and low air pressure
warning devices are operating.
This could include the following:
D.5Build air pressure at high idle, ensure reservoir
air pressure builds from 50 psi (344 kPa) to 90 psi
(621 kPa) within three minutes.
•O
pen the drain valves on all air reservoirs, allow all air
pressure to escape and then drain valves.
D.6Low air pressure warning devices must operate to
a minimum of 60 psi (414 kPa).
D.7Continue building air pressure and check that
the governor places the compressor in the
unloading stage between 115 psi (793 kPa) and
135 psi (931 kPa), (the air pressure gauge will
stop rising). Open the spring parking brake
control valve, releasing the spring parking brakes
(if equipped).
D.8Fan the service brake pedal until the pressure
drops to 90 psi (621 kPa) and no lower than
80 psi (552 kPa). Stop fanning to see if the
governor has placed the compressor in the
loading stage (the reading on the air pressure
gauge should increase.)
D.9Continue fanning the service brake pedal. Low
air pressure warning devices must operate at a
minimum of 60 psi (414 kPa). Build reservoir air
pressure at fast idle to full pressure between
115 psi (793 kPa) and 135 psi (931 kPa).
D.10Release the spring parking brakes if equipped.
Shut off the engine.
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Air Brake Manual
•P
ark the vehicle on level ground, apply the spring parking
brakes, shut off the engine and leave it in gear.
Section summary questions
1.What is the maximum time permitted for the
compressor to build from 50 psi to 90 psi?
2.What is the maximum air pressure loss permitted on
a full brake application with the motor shut off for
combination and single units?
3.How can the holding power of the trailer brakes
be tested?
4.What is the final brake test that should be made before
the vehicle is put into service?
5.Is a brake adjustment part of the air brake test given
by Manitoba Public Insurance?
Air Brake Manual
83
Pre-trip inspection
tear-out sheets
The Air Brake Manual has specific inspection information
that must be studied prior to the test. You may use the
tear-out sheet on page 85 and 86 for your test, but you
cannot add or write on it — if altered, the tear-out sheet
cannot be used on the test.
Key points:
• You need to perform the steps required for the inspection
of various air brake components and tell the examiner
what you’re checking for. Never assume the examiner
has enough information and delete a step. When the
examiner is satisfied in your knowledge of the area you
may be asked to move along.
• The pre-trip inspection should be done in a systematic
manner to conserve time. The air brake pre-trip
inspection should take no more than 30 minutes
to complete.
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Air Brake Manual
Pre-trip inspection points — combination unit
Vehicle Secure
1 Confirm vehicle is secure.
2 Drain air tanks (verbally explain process).
3 Inspect all brake components (if not done in pre-trip).
Compressor and Warning Devices Check
4 Start engine, check to ensure gauges and warning device(s) operate.
5 Ensure compressor is building within required parameters (provide air pressure specifications).
6 Ensure warning devices are operating within required parameters (provide air pressure specifications).
7 Check trailer brake operation.
8 Ensure governor places compressor in the unloading stage (provide minimum/maximum air pressure specifications).
9 Ensure governor places compressor in the loading stage (provide minimum air pressure specification).
10 Ensure warning devices operate at required pressure (provide minimum air pressure specification).
Ensure trailer supply valve operates at required pressure (provide minimum air pressure specification).
Tractor and Trailer Emergency Systems Check
11 Disconnect supply line (explain result).
12 Check trailer supply valve operation. Full air pressure, engine off, trailer valve open (at what pressure does the supply
valve need to close).
13 Build air (provide tolerances), reconnect supply line.
Tractor Protection Valve Check
14 Disconnect control line (explain result).
15 Full air pressure (provide tolerances). Release parking brake, turn engine off.
16 Check tractor protection valve with both valves open (explain result and provide air pressure specifications).
17 Check tractor protection valve by applying hand valve with both valves closed (explain result provide air
pressure specifications).
18 Reconnect glad hands.
19 Build to full pressure, turn engine off.
Air Leak Check
20 Perform leak test.
21 Explain how you’re checking for leaks and provide air pressure specifications.
Brake Adjustment — Combination Unit
22 Both valves open. For auto slack adjusters make 6 full applications 90 psi or higher.
23 Pressure above 90 psi, turn engine off.
Brake Adjustment Verification for Automatic or Manual Slack Adjusters
24 Mark pushrod.
25 Perform a brake application and hold.
26 Measure pushrod travel (provide specific tolerances).
Brake Response Tests
27 Both valves closed, remove blocks.
28 Turn engine on, check to ensure trailer spring brakes will hold the vehicle.
29 Check to ensure tractor spring brakes hold the vehicle.
30 Check to ensure trailer service brakes (only) hold the vehicle.
31 Check service brake response.
Air Brake Manual
85
Pre-trip inspection points —
single unit
Vehicle Secure
Pre-trip inspection points —
air-over-hydraulic
Vehicle Secure
1 Confirm vehicle is secure.
1 Confirm vehicle is secure.
2 Drain air tanks (verbally explain process).
2 Drain air tanks (verbally explain process).
3 Inspect all brake components (if not done in pre-trip).
Compressor and Warning Devices Check
3 Inspect all brake components (if not done in pre-trip).
Compressor and Warning Devices Check
4 Start engine, check to ensure gauges and warning
device(s) operate.
4 Start engine, check to ensure gauges and warning
device(s) operate.
5 Ensure compressor is building within required
parameters (provide air pressure specifications).
5 Ensure compressor is building within required
parameters (provide air pressure specifications).
6 Ensure warning devices are operating within required
parameters (provide air pressure specifications).
6 Ensure warning devices are operating within required
parameters (provide air pressure specifications).
7 Ensure governor places compressor in the
unloading stage (provide minimum/maximum
air pressure specifications).
7 Ensure governor places compressor in the
unloading stage (provide minimum/maximum
air pressure specifications).
8 Ensure governor places compressor in the loading
stage (provide minimum air pressure specification).
8 Ensure governor places compressor in the loading
stage (provide minimum air pressure specification).
9 Ensure warning devices operate at required pressure
(provide minimum air pressure specification).
9 Ensure warning devices operate at required pressure
(provide minimum air pressure specification).
10 Release spring parking brake, turn engine off.
10 Release parking brake, turn engine off.
Air Leak Check
Air Leak Check
11 Perform leak test.
11 Perform leak test.
12 Explain how you’re checking for leaks and provide air
pressure specifications.
12 Explain how you’re checking for leaks and provide air
pressure specifications.
Brake Adjustment — Single Unit
Brake Response Tests
13 Valve open. For auto slack adjusters make 6 full
applications 90 psi or higher.
14 Pressure above 90 psi, turn engine off.
Brake Adjustment Verification for Automatic
or Manual Slack Adjusters
15 Mark pushrod.
16 Perform a brake application and hold.
17 Measure pushrod travel (provide specific tolerances).
Brake Response Tests
18 Valve closed, remove blocks.
19 Turn engine on, check to ensure parking spring brake
will hold the vehicle.
20 Check service brake response.
86
Air Brake Manual
13 Valve closed, remove blocks.
14 Check to ensure parking spring brake holds
the vehicle.
15 Check service brake response.
Metric Conversion Table
A conversion to the metric system took place in Canada on
September 1, 1977, and some commonly used conversions
are listed below. Numbers have been rounded and
therefore are not precise equivalents.
Length
1 inch = 2.54 cm
1 foot = 3
0.48 cm
1 mile = 1.16 km
Volume
1 pint = 0.57 l
kPa to psi
psi to kPa
1 quart = 1.13 l
1 gallon = 4.55 l
5
0.72
1
6.89
10
1.45
2
13.78
15
2.17
3
20.68
20
2.90
4
27.57
25
3.62
5
34.47
30
4.35
6
41.36
35
5.07
7
48.26
40
5.80
8
55.15
45
6.52
9
62.05
50
7.25
10
68.94
60
8.70
15
103.42
70
10.15
20
137.89
Speed
1 mph= 1.61km/h
10 mph= 15
km/h
15 mph= 25
km/h
20 mph= 25
km/h
30 mph= 50
km/h
40 mph= 60
km/h
50 mph= 80
km/h
60 mph=100
km/h
80
11.60
25
172.36
90
13.05
30
206.84
8’ 6”=
2.60 m
100
14.50
35
241.31
10’=
3.00 m
150
21.75
40
275.78
12’ 6”=
3.80 m
200
29.00
45
310.26
13’ 6”=
4.15 m
4.20 m
Converted Linear Measurements
250
36.29
50
344.73
14’=
300
43.51
55
379.20
35’= 11.00 m
310
44.96
60
413.68
38’= 11.50 m
350
50.76
65
448.15
40’= 12.50 m
400
58.01
70
482.62
45’= 14.00 m
415
60.19
75
517.10
60’= 18.50 m
450
65.26
80
551.57
65’= 20.00 m
500
72.51
85
586.04
100’= 30.00 m
550
79.77
90
620.52
500’=150.00 m
585
84.84
95
654.99
600
87.02
100
689.47
650
94.27
105
723.94
10,000 lbs.= 4,500 kg
12,000 lbs.= 5,500 kg
Converted Weight
700
101.52
110
758.41
725
105.15
115
792.89
20,000 lbs.= 9,100 kg
750
108.77
120
827.36
24,000 lbs.=11,000 kg
800
116.03
125
861.83
850
123.28
130
896.31
900
130.53
135
930.78
950
137.78
140
965.25
1000
145.03
145
999.73
1050
152.29
150
1034.20
Abbreviations
mm = millimetre
cm = centimetre
l = litre
km/h = kilometre per hour
m = metre
kg = kilogram
kPa = kilopascal
Air Brake Manual
87
Notes
88
Air Brake Manual
Organ and
Tissue Donation
Every year in Canada, thousands of people wait for organ
and tissue transplants. You can make the difference in
some of those lives. Everyone has the potential to be an
organ and tissue donor — generally, donors are healthy
people of any age who have suffered an irreversible
brain injury, as in a motor vehicle accident or a brain
hemorrhage. Organs and tissues that can be donated
include kidneys, heart, lungs, liver, pancreas, small bowel,
corneas, bones and joints, skin and heart valves.
Here’s what you can do:
• Transplant Manitoba and Tissue Bank Manitoba offer
an on-line organ donation registry at signupforlife. ca.
When you register your intent to be an organ and tissue
donor, your decision is recorded in a Manitoba eHealth
database. At the appropriate time your donation
decision will be shared with your family so they can
honour your decision.
DONOR CARD
Direction under The Human Tissue Gift Act C.C.S.M. c. H180
I,
direct the use, after my death (please check 3) of:
q any needed organs or parts of my body; or
q the following specified organs or parts of my body, namely:
for the following purposes:
q transplant and other therapeutic purposes;
q medical education; q scientific research.
Donor Signature
Date
Co-signature of parent or guardian where donor is under 18 years of age.
Date
• Sign a donor card. Simply cut out the card on the
following page, complete the information, and keep it
with your Manitoba Health Registration Certificate or
driver’s licence so it’s visible for law enforcement and
hospital officials. Note: Drivers under 18 years of age require
the signature of a parent or guardian.
You are encouraged to use the on-line registry even if you
carry a signed donor card. Information on a signed card is
not recorded in a Manitoba Health database and so may
not be available when needed.
It is important that you discuss your wishes regarding
organ and tissue donation with your family. Other
people you may want to inform are your clergy and
family doctor. A Living Will is another place where you
can record your wishes.
Air Brake Manual
89
ORGAN AND TISSUE DONATION
If you wish to donate organ(s) or tissue after death, complete the
attached donor card and KEEP IT WITH YOUR MANITOBA HEALTH
REGISTRATION CERTIFICATE OR DRIVER’S LICENCE SO IT IS VISIBLE
FOR LAW ENFORCEMENT AND HOSPITAL OFFICIALS.
IT IS EXTREMELY IMPORTANT TO DISCUSS YOUR DECISION WITH
YOUR FAMILY TO ENSURE YOUR WISHES ARE FULFILLED.
For more information on organ and tissue donation, please
contact one of the Human Tissue Gift Agencies listed below:
Transplant ManitobaGift of Life Program 204–787–1897
Tissue Bank Manitoba 204–940–1750
Lions Eye Bank of Manitoba and
Northwest Ontario Inc. 204–788–8419
(R-09/09)
90
Air Brake Manual
Notes
Air Brake Manual
91
Notes
92
Air Brake Manual
$10.00
8/15
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