Air Brakes Manual
Air Brake Manual
Foreward
The Air Brake Manual has been prepared by
the Department of Public Safety (Licensing and
Records) 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.
Air Brake Endorsement
· permits the holder to drive vehicles
equipped with air brakes in class
of vehicle for which the driver is licenced.
· To adjust manual slack adjusters,
the operator must hold an
“E” brake endorsement.
Requirements for Air Brake Endorsement
· Must complete an Air Brake Written Test.
· Must complete an Air Brake Practical Test.
The illustrations and explanations of various
types of brake system designs are provided for
instructional purposes only. Most air gauges
measure in imperial units. Therefore 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
Motor Vehicle Act and its regulations.
We gratefully acknowledge the contributions
of all jurisdictions, particularly Manitoba.
www.gnb.ca/0276/index.htm
ISBN 1-55396-034-3
PUBLIC SAFETY
Ce document existe aussi en français.
i
CNB 1674
ii
Table of Contents
Foreward
Foldout i
Air Brake Endorsement
Foldout ii
Requirements for Air Brake Endorsement
Foldout ii
Dual Air Brake System Illustration
Foldout iii
Safety Tips
02
Making Appointments for Tests
03
Section One - Brakes and Braking
05
Heat-Energy-Traction-Friction
06
Speed-Weight-Distance
07
Braking Force
07
Stopping Distance
08
Section Summary Questions
09
Section Two - The Components of an Air Brake System 1 1
The Components of an Air Brake System
12
Compressor and Governor
12
Reservoirs
14
Air Dryer
15
Safety Valve
16
Foot Valve
16
Brake Chambers, Slack Adjusters and Brake Lining 16
Wedge Brakes
20
Disc Brakes
21
Air-Over-Hydraulic Brake Systems
21
Section Summary Questions
24
Section Three - How the Basic System Works
25
Basic Air Brake System
26
One-way Check Valve
26
Air Pressure Gauge
27
Brake Application Gauge
27
Low Pressure Warning Device
27
Stop Light Switch
27
Quick Release Valve
28
Relay Valve
28
Manual Front Brake Limiting Valve
28
Automatic Front Brake Limiting Valve
29
Tandem Rear Axles
30
Section Summary Questions
30
Section Four - Spring Parking Brakes
31
Spring Parking Brake Systems
32
Using a Spring Parking Brake
33
Mechanical Release (Caging)
35
Section Summary Questions
35
Section Five - Trailer System
37
Glad Hands
38
Application Line
38
Trailer Brake Hand Valve
39
Two-way Check Valves
40
Tractor Protection System
41
Tractor Protection Valve
42
Trailer Supply Valve
43
Automatic Trailer Supply Valve System
44
Tractor and Trailer Coupled
46
Charging the Trailer System
47
Foot or Hand Valve Brake Application
47
Emergency Application
48
Supply (Emergency) Line Rupture
49
Control (Service) Line Rupture
49
Loss of Reservoir Air Pressure
50
Manual Trailer Supply Valve
51
Trailer Spring Parking Brakes
52
Section Summary Questions
52
Section Six - Dual Air Brake System
53
Dual Air Brake System with Spring Parking Brakes 56
Spring Parking Brakes with Modulator Valve
57
Combination Tractor and Trailer with Spring Parking
Brakes
58
Section Summary Questions
59
Section Seven - Electronic Controlled Braking
and Traction Systems
61
Anti-lock Brake System (ABS)
62
Automatic Traction Control (ATC)
64
Section Summary Questions
64
Section Eight - Brake Adjustment and In-Service Check 6 5
Brake Adjustment
66
S-cam Brake
66
Stroke vs. Force
67
S-cam Brake Adjustment with Manual Slack Adjuster 68
S-cam Brake with Automatic Slack Adjuster
68
Disc Brake Adjustment
68
Wedge Brake Adjustment
68
After a Brake Adjustment
69
In-service Checks
69
Maintenance and Servicing of the Air Brake System 70
Section Summary Questions
70
Section Nine - Pre-trip Air Brake Inspection
71
Single Unit (Not for air over hydraulic brake systems) 72
Combination Unit
73
Air Over Hydraulic (Air Actuated) Brake System
75
Section Summary Questions
76
Metric Conversion Table
77
1
Safety Tips
1. Reminder - is your commercial trailer equipped
with the mandatory retro-reflective markings? In
January 2002, under the motor vehicle inspection
program, all trailers must now be equipped with
retro-reflective markings. Be seen - be safe.
an eye on a young child in the vehicle increases the
risk of being involved in a collision. All drivers
should drive defensively and be prepared for the
unsafe actions of other motorists or for poor driving
conditions. Expect the unexpected.
7. Operation Lifesaver reminds all drivers to stay
alert at all times and especially when crossing a
railway track.
2. Seatbelts Save Lives - Please Buckle Up - The Life
you Save May Be Your Own
- Be careful - low slung trailer units can get stuck on
raised crossings.
The proper use of occupant restraints has become
the most cost-effective method to reduce death and
injuries resulting from motor vehicle collisions.
- Know the length of your truck and trailer. When
you see a signal or stop sign be certain you have
enough room to completely clear the railway tracks
before crossing.
3. Animals on the Highways - Slow Down - Please
Be Alert.
- Drivers should use caution especially at dawn and
dusk when the animals are on the move.
- Animals are unpredictable so reduce your speed.
- Stay alert and scan both sides of the road, not just
the pavement in front of your vehicle.
4. Cellular Phones - Cellular telephones are an
important safety aid for drivers. Many people use
their cellular telephone to report accidents and
crimes and for their personal safety when their
vehicle breaks down or they are lost.
- Use a hands-free device to make it easier to keep
both hands on the wheel.
- When dialling manually, dial only when stopped, or
have a passenger dial for you.
- Avoid unnecessary calls and keep conversations
to a minimum.
- Be familiar with the various functions of your
cellular phone and program frequently dialled
numbers.
- Do not use your cellular phone when driving
conditions are hazardous.
- Remember it is an offence under the Motor Vehicle
Act to drive without due care and attention.
5. SEE AND BE SEEN! - Turn on your headlights.
6. DRIVER DISTRACTIONS - Many everyday habits
of drivers are dangerous and can lead to crashes.
Distractions such as eating fast food, drinking
coffee, changing the radio station, switching CDs or
tapes, talking on a cellular phone or trying to keep
2
Take Care of Yourself!
The most important part of a moving truck or bus is
the driver! Get plenty of rest before getting behind
the wheel. Eat well and stay fit. Remember, hours of
service violations are serious and can threaten your
livelihood or even your life. Stay healthy and well
rested, or don’t drive.
Always Maintain Your Vehicle
Inspect your vehicle before each trip and check
your brakes regularly. Learn how to inspect your
brakes, identify safety defects, and get them
repaired before risking your life and others on the
highway.
Slow Down in Work Zones
Watch out for highway construction. Stay alert.
Work zone crashes are more likely to happen during
the day. Almost one-third of fatal crashes in work
zones involved large trucks. Take your time going
through work zones and give yourself plenty of
room. Expect the unexpected.
Always Keep Your Distance
Always leave enough space between you and the
vehicle in front of you. If you hit someone from
behind, you are typically considered “at fault”,
regardless of the situation. Large trucks require
more stopping distances than other vehicles. Take
advantage of your driving height, and anticipate
braking situations.
Always Drive Defensively
Avoid aggressive drivers! It is estimated that each
year, two-thirds of all traffic fatalities are caused by
aggressive driving behaviours Keep your distance
and maintain a safe speed. The only thing speed will
increase is your chance for a crash.
Note:
Work to Help Yourselves
· Do not mismatch air chamber in size
Be the professional on the highway and at safety
events! Help stranded motorists; notify traffic safety
agencies of crashes, unsafe drivers, unsafe
roadway conditions, and other situations that can
lead to crashes. your participation in public safety
events and your performance on the highway can
change public perception!
YOU RARELY RUN OUT OF BRAKES, BUT YOU RUN
OUT OF ADJUSTMENT. (The brake components
could all be new but if the adjustment is not done,
the brakes will not do their job.)
Check the steering brake air line - it’s well worth the
time. It is recommended that the airline that feeds
the steering brakes be inspected for bulges, flat
spots, cracks and looseness at the fitting. This is an
important safety issue as a blown airline hose will
result in rapid loss of air pressure and decreased
ability to stop.
· Make sure that your brakes are
properly adjusted
on the same axle.
· A properly installed air chamber and
slack adjuster should not have more
than a 90 degree angle between the
components.
· Do not mismatch slack adjusters in
length on the same axle.
Making Appointments
for Tests
Contact your local Service New Brunswick office
to arrange for an appointment and any additional
information regarding testing procedures.
Ensuring proper brake operation and safety is the
responsibility of the driver. Take time during the pretrip inspection to check the brakes - it could prevent
a serious collision.
Brake related defects continue to be the most
frequent reason commercial vehicles are put out-ofservice. The driver/carrier can make a difference by
a) increasing knowledge of brake compliance and
vehicle brake performance, and
b) making sure all applicable brake system
inspection requirements are followed.
3
4
SECTION ONE -
BRAKES AND BRAKING
5
Heat-Energy-Traction-Friction
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.
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 1/10 the time it took to
reach 100 km/h).
100 km/h
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.
10X
To stop the vehicle in 1/10 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 1/6
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
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.
Delivered force
= 400 lb
Applied force
= 100 lb
A
C
For example, a vehicle carrying a load of 14,000 kg at
16 km/h is brought to a stop in 30 metres 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
metres. This would be more braking force than the
brakes could provide. No vehicle has enough
braking force when it exceeds its limitations.
B
If a 100 lb downward force is applied at point A, then
the upward force at point B is 400 lb.
Braking Force
Mechanical
Braking systems use devices to gain a mechanical
advantage. The most common device for this
purpose is leverage.
E
R
B = 400 lb
A lever is placed on a pivot called the fulcrum. 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.
Look at this simple lever system:
S-cam brake
4 feet
A
100 lb
1 foot
A
B
C
C
B
400 lb
B
400 lb
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 push
rods. If compressed air of 120 psi acts on a
diaphragm of 30 square inches, 3,600 lb of force is
produced (120 x 30). Apply this force to a push rod to
move a 6-inch slack adjuster operating a cam and
the total force equals 21,600 inch pounds torque
(3,600 x 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
Stopping Distance
Stopping distance consists of three factors:
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.
8
· 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,
approximately 3/4 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
4/10 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.
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
car
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?
9.
What is compressed air?
10. What does the abbreviation “psi” stand for?
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?
Loaded
truck
“Driver’s Reaction Time” - “Braking Distance” “Brake Lag.”
Brakes applied
Actual stop
Actual stop
9
10
SECTION TWO -
THE COMPONENTS
OF AN AIR
BRAKE SYSTEM
11
Section One of this manual has 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 Two
will explain 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.
The 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.
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). 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
5. Brake linings and drums or rotors to create the
to do so the compressor requires servicing. A
friction required to stop the wheels.
compressor may not be able to build air pressure from 50
It is necessary to understand how each of these
to 90 psi within three minutes if the air filter is plugged or
components work before studying their functions in
if the belt is slipping. If these were not at fault the
the air brake system.
compressor could be faulty.
Governor
Pressure setting spring
Exhaust port
Unload port
Reservoir port
12
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, 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.
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.
· Intake 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.
Compressor (Unloading stage)
Compressor (Intake stroke)
Inlet
valve
Inlet
valve
Unload
plunger
Unload
plunger
Piston
Piston
Intake air filter
Intake air filter
Discharge valve
From governor
Discharge
valve
Usually compressors are lubricated from the engine
lubrication system, although some compressors are
self-lubricating and require regular checks of the
lubricant level.
13
· Compression 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)
Inlet
valve
To reservoir
Unload plunger
Piston
Intake air filter
Discharge
valve
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. Most 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,
14
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. Allow all air pressure to escape, which will
then permit the moisture collected in the reservoir to
drain.
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.
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.
Reservoir
Air Dryer
Air Dryer (Purge cycle)
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.
Control
port
Supply
port
Cut-off piston
Purge
valve
Exhaust
Delivery
port
One-way
check valve
Air Dryer (Drying cycle)
One-way check valve
Orifice
Oil Separator
Desiccant bed
Desiccant
Cartridge
Control Port
Dried Air
Air Dryer
Supply Port
Check valve
assembly
Control Port
Cut-off
piston
Reservoir
Supply Port
Compressor
Delivery Port
Governor
Purge valve
Exhaust
Heater
element
Sump
15
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 springloaded 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
Foot Valve
Treadle
Treadle spring
Exhaust Port
To brake chambers
To brake chambers
Supply from reservoir
When the driver applies the brakes, depressing the
treadle part way, 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.
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.
Brake Chambers, Slack Adjusters and
Brake Lining
Brake Chamber and Slack Adjuster (Brakes off)
Air inlet
Brake chamber
Foot Valve
Mounting
bolts
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.
Slack
adjuster
Diaphragm
16
Clevis and pin
Diaphragm return
spring
Push rod
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.
Brake Chamber and Slack Adjuster (Brakes on)
Air inlet
Brake chamber
Mounting
bolts
Clevis and pin
Push rod
Slack
adjuster
Diaphragm
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 push rod. The push
rod is connected by a clevis and pin to a crank armtype lever called a “slack adjuster.” This converts
the pushing motion of the push rod 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 push rod to the released position.
As indicated by its name, the slack adjuster adjusts
the “slack” or free play in the linkage between the
push rod 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.
Diaphragm return
spring
Manual Slack Adjusters
Ball Indent Slack Adjuster
Positive Lock Slack Adjuster
Lock screw
Locking collar
Worm shaft
Adjusting bolt
Adjusting bolt
Worm gear
Grease fitting
Spline
Spline
17
Previously illustrated are two common types of
manual slack adjusters, showing the worm adjusting
gear. When the brakes are fully applied, the angle
between the push rod and the arm of the slack
adjuster should be no more than 90° (at a right
angle).
Brake Chamber and Slack Adjuster (Brakes on)
Air
inlet
Pushrod
90 °
Slack
adjuster
On manual slack adjusters, the adjusting worm bolt
is turned until the brake linings touch the drums and
then backed off, normally ˘ to ˚ 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. 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.
When conducting a pre-trip air brake inspection look
for worn or damaged components, also ensure that the
slack adjuster and push rod are at 90° with the brakes
applied, as illustrated. If more than 90° there is a drastic
loss in braking efficiency, less than 90° may indicate an
over adjustment and brakes could be dragging.
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 push rod travel to maintain the same braking
force. If a brake is out of adjustment there would not be
enough reserve stroke of the push rod travel to
compensate for drum expansion. This would cause a
brake fade and would greatly extend stopping distance.
If travelling down a hill, this could cause complete
brake loss.
Note: Detailed brake adjustment procedures are outlined
in Section Eight.
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 gear
18
Worm seal
Adjusting bolt
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 strokesensing 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 push rods. Even poor visual
inspection can result in problems unrelated to
adjuster function. 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. As an automatic slack adjuster ages in service,
the components wear that sense when an adjustment
is required. 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
Brake Assembly
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.
However it would not take long in this severe braking
condition for one or more of the brake drums to over
heat and expand. The over heating will physically
increase the brake drums diameter, and in extreme and
prolonged conditions will lead to longer push-rod
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 per day may be
required to keep the automatic brake adjusters in
adjustment (see page 68 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.
Illustrated is 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 chamber
Push rod, clevis and pin
Slack adjuster
S-cam
Brake lining
Brake drum
19
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 brake chamber push rod forces a wedge-shaped
push rod between the brake shoe rollers. This
Wedge Brake - Single Chamber
Brake lining
Brake chamber
The twisting action of the brake cam shaft and Scam 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.
Wedge Brakes
This is another example of a brake assembly used
on some air brake-equipped vehicles. The action of
Single chamber
Brake shoe roller
Shoe return spring
Brake shoe
Push rod
Adjusting wheel
forces the brake shoe lining against the brake drum.
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.
Wedge Brakes
Dual chamber
Brake lining
Brake
chambers
Brake chamber
Shoe return
springs
Adjusting wheel
20
Adjusting wheel
Disc Brakes
Air-Over-Hydraulic Brake Systems
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 were developed
for medium weight vehicles because:
Disc Brake
· 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 Actuated Hydraulic Brake System
(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.
21
Air-actuated Hydraulic Brake System
Air brake
chamber
Foot valve
Air lines
Hydraulic lines
Hydraulic master
cylinder
Hydraulic
wheel
cylinders
Hydraulic wheel
cylinders
Hydraulic master
cylinder
Compressor
Reservoirs
Air lines
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 Nine; Pre-Trip Air Brake
Inspection).
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.
22
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 found usually 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.
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.
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.
Air-boost Hydraulic Brake System
Air
lines
Reservoir
Booster unit
Hydraulic master
cylinder
Compressor
Hydraulic
line
Hydraulic
wheel
cylinders
Hydraulic
wheel
cylinders
Brake
pedal
Air lines
Booster unit
Hydraulic line
23
Section Summary Questions
1.
2.
3.
4.
What are the five basic components of an air
brake system?
At what pressure should the governor cause the
compressor to return to its “loading” stage?
At what pressure will the governor place the
compressor in the “unloading” stage?
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 push rod travel adjusted within
limitations?
17. What is the most common cause of loss of
effective braking in an air brake system?
24
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?
SECTION THREE -
HOW THE BASIC
SYSTEM WORKS
25
Basic Air Brake System
One-way Check Valve
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 push rods move the
slack adjusters. The slack adjusters rotate the Scams, 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.
Spring
Body
Ball
Cap nut
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.
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.
Note: An air dryer (3) has been added to reduce the
amount of moisture in the system.
One-way Check Valve
In the diagram below, two reservoirs are shown
(5)(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
Basic Air Brake System
32
31
11
2
32
26
1 3
4
5
7
9 10
11
Air Pressure Gauge
Low Pressure Warning Device
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.
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
must be comprised of two systems - visual and audible
- consisting of a red warning light and 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 55
psi. Some vehicles are equipped with both a light and
a buzzer to warn the driver of a low air pressure
condition.
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 55 psi. The
automatic warning device will rise out of the driver’s
view when the pressure in the system rises above 55
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 55 psi.
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.)
Whichever warning system is used, buzzer-lights or
wig-wag, the driver must stop the vehicle and find the
cause of the air loss. The air pressure remaining in the
system (approximately 55 psi) is enough for a brake
application if the driver acts promptly.
Stop Light Switch
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.
27
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. To allow
the brakes to release quickly and fully by
discharging the application air near the brake
chambers, a quick release valve (33) may be
installed.
the foot 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.
Relay Valve
Quick Release Valve
Manual Front Brake Limiting Valve
For better steering control 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.
Relay 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 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
Dash Mounted
Control Valve
Service port
Lever
From
limiting
valve
To limiting valve
Exhaust port
Service port
Delivery ports not shown
28
Basic Air Brake System with Manual Front Brake Limiting Valve
29
32
35
11
31
13
36
32
1
3
4
2
6
7
6 8
9
11
5
Automatic Front Brake Limiting Valve
Service port
Service port
Piston spring
Lower piston
assembly
Delivery port
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
Inlet-exhaust valve
spring
Delivery port
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.
29
Tandem Rear Axles
34
33
13
The air brake system discussed previously is for a
vehicle with a single rear axle. The diagram illustrates
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.
Section Summary Questions
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.
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
SECTION FOUR -
SPRING PARKING
BRAKES
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
brake- equipped 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. A 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, depending on how the
system is piped.
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 below
approximately 35 psi in the system. Any time the
reservoir pressure drops to approximately 35 psi,
this valve will exhaust automatically, placing the
spring parking brakes into full application. On some
older vehicles there may be a single type of pushpull 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
Mounting Bolts
Control valves will vary, depending on the
manufacturer and type of piping arrangements.
Clevis and pin
Dust cap
Slack
adjuster
Spring parking
brake chamber
Push rod
Diaphragm
return
spring
32
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.
Service Brakes Applied
Brake On
Service brake
chamber
Parking brake
spring
Mounting Bolts
Slack
adjuster
Clevis and pin
Dust
cap
Push rod
Diaphragm
return
spring
Diaphragm
Spring parking
brake chamber
Using a Spring Parking Brake
27
12
12
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.
33
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.
Spring Parking Brakes Applied
Brakes On
Service brake
chamber
Parking brake
spring
Mounting Bolts
Slack
adjuster
Clevis and pin
Dust
cap
Push rod
Diaphragm
return
spring
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 brakes and the service brake.
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; and,
· the adjustment of the service brakes.
34
Diaphragm
Spring parking
brake chamber
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 over heated 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. 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.
Parking Brake Spring Caged
Brakes Off
Service brake
chamber
Parking brake
spring
Mounting Bolts
Caging
bolt
Clevis and pin
Slack
adjuster
Push rod
Diaphragm
return
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.
Diaphragm
Spring parking
brake chamber
Section Summary Questions
1.
What is meant by “compounding” the brakes?
2.
Why are spring brakes a reliable type of parking
brake?
3.
6.
What is the reason for releasing the parking
brakes before making a full brake application
test?
How are parking brakes held in the released
position?
7.
Why must you be careful parking a vehicle with
overheated brakes?
4.
What are the functions of the cab-mounted
parking brake control valve?
8.
How can some types of parking brakes be
released without the use of air pressure?
5.
Will parking brakes apply “automatically” in
all braking systems?
9.
What is the danger of disassembling a spring
parking brake unit?
35
36
SECTION FIVE -
TRAILER SYSTEM
37
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.
locked to the glad hand of the supply (emergency)
line to keep water and dirt from entering the unused
lines. The cleaner the air supply is kept, the less
chance of brake problems.
In the following pages the power unit of a combination
vehicle will be referred to as a tractor.
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.
Glad Hands
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
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:
· if the trailer broke away from the tractor, the
trailer would not have brakes.
· if the control (service) line parted or ruptured, the
Rubber seal
trailer brakes would not be applied, and the
application air would be lost from the tractor if the
brakes were applied.
· if the air pressure in the reservoirs is lost, there
would be no way to apply the brakes of the tractor
or the trailer.
· the trailer brakes cannot be applied independently
Air line
connect together and lock in position. They have
rubber gaskets that prevent air from escaping at the
connections.
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. Vehicles
equipped with “dead-end” couplers should use them
whenever the vehicle is used without a trailer to
prevent water and dirt from entering the coupler and
lines.
If the unit is not equipped with dead-end couplers,
the glad hand of the control (service) line can be
38
from the tractor and there is no way to set the
trailer brakes when coupling to the tractor.
· the application and release of the trailer brakes
would be slower than those of the tractor.
These disadvantages are overcome by the addition
of the supply (emergency) line and valves discussed
in the following pages.
The illustration 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
30
31
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.
39
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.
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 hand-operated
valve.
Air from
hand valve
Air from foot
valve
Shuttle
To trailer
Foot Valve Application
30
26
31
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 two-way check valve (26). The shuttle has
shifted to the low pressure side, closing off any air
40
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.
Hand Valve Application
30
26
31
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. 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.
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
parking control” 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. The
other is 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.
41
Tractor Protection Valve
To the supply
(emergency)
glad hand
To the control
(service) glad
hand
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.
Tractor Protection System (open)
(trailer charged)
Air from foot
valve
It also has two air lines: one coming from the supply
(emergency) line (21), and one coming from the control
(service) line (22). The tractor protection valve is
spring-loaded 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. 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 on the tractor
causing the tractor protection valve to close, which in
turn closes off 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.
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.
Tractor protection valve (24)
Two-way
check valve
(26)
Control
(service) line
(22)
Air from hand
valve
Air from
reservoir
Supply
(emergency)
line
(21)
Trailer supply valve
(mounted in cab)
(28)
42
Tractor Protection System (closed)
(trailer not charged)
Air from foot
valve
Tractor protection valve (24)
Two-way
check valve
(26)
Control
(service) line
(22)
Air from hand
valve
Air from
reservoir
Supply
(emergency)
line
(21)
Trailer supply valve
(mounted in cab)
(28)
To test the proper function of the tractor protection
valve, hook the tractor’s air lines to a trailer, ensure
the vehicle is secure and the wheels are blocked and
release the parking brake. Verify that the system is at
full pressure, charge the trailer by opening the trailer
supply valve, make a brake application and hold it,
(use a hand brake valve if alone.) Disconnect the
control (service) line (there will be a loss of air.) Now
disconnect the supply (emergency) line. Control
(service) air will stop immediately and supply (emergency) air should continue to be lost, depending on
the system.
Reconnect the supply (emergency) line and again
service air will be lost.
This tests the proper opening and closing operation of
the spring portion of the 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.
Trailer Supply Valve
This valve (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.
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, opening the exhaust port. The driver can
close the valve manually. This will open the exhaust
port without the pressure being depleted from the
tractor reservoirs. The spring parking brakes will also
be applied.
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.
43
Automatic Trailer Supply Valve System
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).
The diagram below illustrates 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
28
26
22
21
24
20
In the diagrams the upper line is the control (service)
line (22) and the lower line is the supply (emergency)
line (21).
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.
Illustrated is a tractor equipped with a trailer supply
valve (28) and a tractor protection valve (24). The
22
30
28
21
26
24
20
31
44
30
26
28
24
20
31
In this illustration, 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
tractor protection valve (24).
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.
30
28
There is no air loss from the tractor through the
disconnected glad hands (20).
26
24
20
31
45
Tractor and Trailer Coupled
28
20
6
16
39
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
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. It 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.
46
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 parted air lines between
tractor and trailer, or loss of air pressure from the
power unit service system. A ruptured control
(service) line would not cause an emergency brake
application on the trailer until a brake application
was made, and this would cause a rapid loss of air
pressure from the system. 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 reservoir from going back to the
source of the supply.
Charging the Trailer System
24
28
39
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 be open when the tractor pressure has
built up to approximately 90 psi, depending on the
valve.
Foot or Hand Valve Brake Application
26
24
30
31
This and the next illustration are to only 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.
47
30
14
31
16
39
14
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.
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.
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.
Emergency Application
22
21
14
16
39
14
48
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.
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
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.
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.
21
14
16
39
14
Control (Service) Line Rupture
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. 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.
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
brake action. 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.
49
Control (Service) Line Rupture
22
31
Loss of Reservoir Air Pressure
5
7
8
Rupture of the compressor discharge line would
result in loss of air pressure from the supply/wet
reservoir. 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 diagram, 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.
50
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.)
Loss of Reservoir Air Pressure
28
21
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.
The truck protection system described is an
example of a tractor equipped with a type of cabmounted 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.
Manual Trailer Supply Valve
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.
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 the 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.
51
Trailer Spring Parking Brakes
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.
A trailer with spring parking brakes has these
components:
· front and rear service reservoirs (16)
· trailer spring parking brake valve (18)
· relay 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 parking brake valve is responsible
for several important functions:
1) It 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 (ie: trailer
breakaway).
22
18
17
21
15
16
Section Summary Questions
10. Describe the function of the control (service) line.
1.
What is the purpose of a two-way check valve?
2.
Why should the glad hands be protected when not
in use?
11. What will occur if the supply (emergency) line
ruptures?
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.
52
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
parking brake valve?
SECTION SIX -
DUAL AIR
BRAKE SYSTEM
53
More and more heavy duty 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
Basic Dual Air Brake System
54
the basic air brake system described so far, and if
the dual system is separated into basic functions, it
becomes quite simple.
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 twoaxle 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.
Dual Air Brake System
29
31
1
4
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 oneway check valves (7). At this point, the dual circuits
start. Air from the primary/dry reservoir is directed to
the foot valve (31). 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.
5
7
8
9
10
Dual-circuit Foot Valve
Treadle
Supply
ports
Delivery
ports
Exhaust port
55
Dual Air Brake System with Spring Parking Brakes
27
26
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
56
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.
Spring Parking Brakes with Modulator Valve
27
23
12
12
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.
57
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 two-way 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 parking brake valve (18) directly into the
trailer spring parking brake chambers (15). When air
enters, the pressure protection part of the trailer
spring parking 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 tractor breaks away from the trailer, the control
(service) and supply (emergency) lines will be pulled
apart. The sudden loss of air in the supply (emergency) line 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
28
26
24
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 recoupled 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, causing 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.
18
15
17
15
58
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 parking
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?
59
60
SECTION SEVEN -
ELECTRONIC CONTROLLED
BRAKING AND TRACTION
SYSTEMS
61
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 ice
slicked roads, through curves or during lane
changes. The air brake system remains the same
with the addition of the ABS components.
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.
An ABS basically consists of:
· An electronic control unit (ECU)
· A wheel sensor and tooth wheel
· ABS valves
Tooth Wheel and Sensor Block
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.
For information specific to the vehicle you drive, refer
to the operator’s manual.
Axle
Wheel
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 they
may have 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 ABS.
62
Four sensor - four modulator valve anti-lock brake system
Relay block
Tooth wheel and
sensor block
Modulator
valves
Brake
chambers
E.C.U.
Tooth wheel and
sensor block
Tooth wheel and
sensor block
Dash lamp
Brake
chambers
Brake
chambers
Brake pedal
Modulator
valves
Tooth wheel and
sensor block
63
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?
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?
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
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.
64
SECTION EIGHT -
BRAKE ADJUSTMENT AND
IN-SERVICE CHECK
65
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 outwards. 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 (push rod travel) must be checked
as part of the pre-trip air brake inspection (Section
9).
Brake off
Chalk mark
Brake on
Chalk mark
S-cam Brake
The following are recommended steps to determine
if an S-cam brake with manual or automatic slack
adjusters requires adjustment.
· Ensure vehicle is secure and wheels are blocked.
· Shut off the engine, leaving the transmission in a
Ruler
low gear or park then release the spring parking
brakes.
· Make a chalk mark where each push rod enters
the brake chamber.
· Reapply the spring parking brakes and measure
the distance from the brake chamber to the
chalk mark. Ensure the slack (push rod travel) is
within 3/4 and 1 1/2 inches or within
manufacturer’s specifications and the angle
between the slack adjuster and push rod is 90°
or as close as practical. If not, then a brake
adjustment is required.
When the brakes are out of adjustment, braking
efficiency is reduced by three factors:
1. Brake lag increases because additional air is
required to fill and pressurize the increased
chamber volume caused by the increased stroke
of the push rod.
2. The angle between the slack adjuster arm and the
push rod becomes more than 90°, which results in
a loss of force between the linings and
the drum. (see diagram on page 18)
3. The effectiveness of diaphragm brake chambers
drops off significantly if the stroke exceeds 75% of
its designed travel. For a type 30 chamber (30
square inches of effective diaphragm area) that
has a design stroke of 2 1/2 inches, the brakes
should be adjusted at or before 1 1/2 inches of
travel. With a working pressure of 100 psi this
chamber will produce a force of 3,000 lb at 1 1/2
inches of travel, but only 2,500 lb of force at a
travel of 2 1/4 inches. Remember, when a chamber
bottoms out, the force reduces to zero.
66
Other factors that affect the braking ability of the
vehicle, include:
·
·
·
·
·
·
overheated brakes;
worn linings;
oversize drums;
malfunctioning valves;
valves with above normal release pressure;
seized or partially seized brake camshafts or shoe
anchors;
Brake failures are seldom the result of catastrophic
system failure. They are usually the result of brakes
being out of adjustment, often to the point that there
is no braking action on some wheels.
Under frequent braking conditions and depending
on the severity, there will be some expansion of the
brake drum because of heat and the amount will
vary with the thickness of the drum. This expansion
of the drum will also add to the overall push rod
travel.
The driver should be trained in brake adjustment
prior to attempting the practical examination.
· excessive moisture in reservoirs;
· contaminated brake linings.
Stroke vs. Force
The amount of force available at the push rod is
consistent out to two inches of stroke. After two
inches, push-rod force drops very quickly.
4000
Push-rod Force (in lb) at 100 psi
Recommended max. stroke at
100 psi before readjustment
3000
2000
1000
Air chamber
bottom-out
0
0
1/2
1
1 - 1/2
2
2 - 1/2
3
Push-rod Stroke
(in inches)
Don’t be Fooled - Check the Slack
It is up to you, the driver, to ensure that your vehicle
has safe, properly adjusted brakes.
67
S-cam Brake Adjustment with Manual Slack Adjuster
· Ensure vehicle is secure and wheels blocked,
release the parking brake.
· Verify system is at full pressure.
· Use a proper wrench to disengage the external
locking device (if so equipped) from the adjustment
bolt of the slack adjuster.
· Turn the adjustment bolt until the lining contacts the
drum and the adjustment bolt will not turn any
further. If possible, visually check to see that the
brake linings are in contact with the brake drum.
Pull or pry on the slack adjuster. There should be no
movement or free play. If there is free play, you have
turned the adjusting nut the wrong way.
· When turning the adjustment bolt on the slack
adjuster, the S-cam should turn in the same
direction as if a brake application were being
made.
· Back off the adjustment bolt about 1/4 to 1/2 of a
turn and ensure the locking device reengages.
· Now recheck slack adjuster travel. The travel
should now be between 1/2 to 3/4 inches free travel
while pulling on the linkage, or within
manufacturer’s specifications.
Note: Applicants conducting an air brake examination
will be required to satisfactorily adjust S-cam brakes
with manual slack adjusters.
The equipment required to adjust a brake must be
supplied by the applicant.
S-cam Brake with Automatic Slack Adjuster
Automatic slack adjusters adjust themselves during
brake applications to accommodate for brake lining
and drum wear. However, they must be checked
daily to ensure they are maintaining proper push rod
travel which normally is two inches when the brake
is applied. Normally two to four brake applications
of 80-100 psi per day will keep the brakes properly
adjusted, a driver may be required to consciously
make these applications when starting out for the
day or during the trip when possible. If they are
badly out of adjustment, a qualified mechanic should
inspect them. Adjusting automatic slack adjusters
by hand is not recommended unless you are
thoroughly trained on the proper adjustment
procedures as dictated by the manufacturer. If an
automatic slack adjuster requires adjusting by hand
the slack adjuster may not be working properly and
should be inspected or replaced by a qualified
mechanic as soon as possible.
Disc Brake Adjustment
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.
Wedge Brake Adjustment
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
68
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.
· 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.
· Repeat for the other shoe on that brake assembly.
· Replace the dust covers in the adjusting slots.
· Repeat the above steps on the other brake
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 air 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 brakes at each wheel.
assemblies.
In-service Checks
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:
· the compressor is maintaining full reservoir air
pressure;
· push rod travel is within limitations on
all chambers;
69
·
·
·
·
there are no air leaks;
Section Summary Questions
the glad hands and the lines are secure;
1.
Should all drivers be able to adjust S-cam
brake systems?
2.
This should be used as an in-service check only and
not mistaken as a daily pre-trip air brake inspection.
What tests must be made at a “truck stop here”
sign posted before a steep or long downgrade?
3.
The driver must be aware of the condition of the
vehicle’s braking system at all times. This can be
done by:
Do the brake adjustment specifications differ
between the S-cam and the disc brakes
system?
4.
After an adjustment has been made when
should the brakes be checked?
5.
What is brake fade?
6.
What is the main cause of brake failure?
7.
What are three ways to check the condition of a
vehicle’s braking system?
the drums, bearings and tires are not overheating;
the trailer supply valve is operating properly.
· watching the pressure gauges;
· hearing the warning signals; and
· feeling the braking response of the vehicle.
By these methods 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.
Maintenance and Servicing of 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
operating condition before the vehicle moves.
70
SECTION NINE -
PRE-TRIP AIR BRAKE
INSPECTION
71
Single Unit (Not for air over hydraulic brake
systems)
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.
The pre-trip air brake inspection should be completed
within 20 minutes. Failure to complete the test within
20 minutes may result in the test being discontinued.
· Fan the brake pedal until the pressure drops to 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).
· Continue fanning the brake pedal. Warning
device(s) must operate at or before 55 psi and the
spring parking brake should apply at or before the
reservoir pressure drops to 20 psi.
Check for Leakage
Practical examination will be conducted in either the
metric or imperial systems of measurement,
whichever coincides with the equipment being
operated.
· Rebuild reservoir air pressure at fast idle. When
Vehicle Secure
· Shut off the engine.
· Make a full brake application with the foot valve
· Set the spring parking brake on the vehicle.
· Block the wheels; the vehicle should be on level
ground if possible.
· Check that the compressor is secure.
· Check the drive belt and pulleys for wear, cracking,
slippage and tension (if equipped).
· Drain all reservoirs completely (wet/supply reservoir
first). When reservoirs are empty close the drain
valves. “For testing purposes only, the applicant will
not be required to manually drain the reservoirs.
Instead, the applicant will verbally explain the steps
that they must follow when draining the reservoirs
and then fan the brake pedal to empty the
reservoir.”
· Check brake chambers, air lines and slack adjusters
for security and wear.
Compressor and Warning Devices Check
· Start the engine and run at fast idle to build air
pressure.
· Warning device(s) must operate to 55 psi or higher.
· Ensure reservoir air pressure builds from 50 to 90 psi
within three minutes.
· Release the spring parking brake at 90 psi or above
to prevent compounding the brake pressure.
· Continue building reservoir air pressure. Check that
the governor places the compressor in the
unloading stage between 115 psi minimum and 135
psi maximum.
72
the pressure reaches 90 psi release the spring
parking brake. Continue to build pressure to
between 115 psi minimum and 135 psi maximum.
and hold. Observe the reservoir air pressure
gauge.
· The reading should not drop more than 3 psi per
minute. Lightly tap the gauge to ensure it is not
sticking. For an accurate reading brake
application should be held for at least two
minutes.
· Listen for audible air leaks.
· Release full brake application.
Brake Adjustment
(A driver should be trained in brake adjustment prior
to attempting practical examination and supply the
equipment required to do a brake adjustment.)
The following are recommended steps to determine
if an S-cam brake with manual slack adjusters
requires adjustment.
· Ensure vehicle is secure and wheels are blocked.
Verify the system is at full pressure.
· Shut off the engine. Leaving the transmission in a
low gear or park.
· Release the spring parking brakes.
· Make a chalk mark where the push rods enter the
brake chambers.
· Reapply spring parking brakes and measure the
distance from the brake chamber to the chalk
mark. Ensure the slack (push rod travel) is within
1/2 - 3/4 inches or within manufacturer’s
specifications and the angle between the slack
adjuster and push rod is 90° or as close as
practical. If not, then a brake adjustment is
required.
Combination Unit
When upgrading your driver’s licence from single to
combination vehicles with air brakes, you will be
required to satisfactorily demonstrate the pre-trip air
brake inspection for combination vehicles.
The pre-trip air brake inspection should be completed
within 30 minutes. Failure to complete the test within
30 minutes may result in the test being discontinued.
The following are recommended adjustment steps
for S-cam brakes with manual slack adjusters.
Practical examination will be conducted in either the
metric or imperial systems of measurement,
whichever coincides with the equipment being
operated.
· Ensure vehicle is secure and wheels blocked,
Vehicle Secure
release the spring parking brake.
· Verify reservoirs are at full pressure.
· Use a wrench to disengage the external locking
device (if so equipped) from the adjustment bolt of
the slack adjuster.
· Turn the adjustment bolt until the brake lining
contacts the brake drum. If possible, visually
check to see that the brake linings are in contact
with the brake drum.
· When turning the adjustment bolt on the slack
adjuster, the S-cam should turn in the same
direction as if a brake application were being
made.
· Back off the adjustment bolt about 1/4 to 1/2 of a
turn. Check that the push rod free travel is now
between 1/2 to 3/4 inches, or within
manufacturer’s specifications.
Brake Test
· Reapply the spring parking brake, remove wheel
blocks.
· Gently tug against the spring parking brakes in
low gear, it should hold the vehicle.
· Release the spring parking brakes.
· Move vehicle ahead slowly and make a service
brake application to check brake response.
· Set the spring parking brake on tractor and trailer.
· Block the wheels; the vehicle should be on level
ground if possible.
· Check that the compressor is secure.
· Check the drive belt and pulleys for wear, cracking,
slippage and tension (if equipped).
· Drain all reservoirs completely on the tractor
(wet/supply reservoirs first) then the trailer.
When reservoirs are empty close the drain valves.
“For testing purposes only, the applicant will not be
required to manually drain the reservoirs. Instead,
the applicant will verbally explain the steps that they
would follow when draining the reservoirs and then
fan the brake pedal to empty the reservoir.”
· Check brake chambers, air lines and slack adjusters
for security and wear.
· Ensure the air brake glad hands and trailer electrical
cord is connected properly.
Compressor and Warning Devices Check
· Start the engine and run at fast idle to build up air
pressure.
· Warning device(s) must operate to 55 psi or higher.
· Ensure reservoir air pressure builds from 50 to 90 psi
within three minutes.
73
· At 90 psi or above, release the spring parking
brakes to prevent compounding the brake
pressure and charge the trailer system by opening
the trailer supply valve. Apply and release the
trailer brakes to ensure they are releasing.
(Failure to release could indicate supply
(emergency) and control (service) lines are
crossed).
· Continue building reservoir air pressure. Check
that the governor places the compressor in the
unloading stage between 115 psi minimum and
135 psi maximum.
· Fan the brake pedal until the pressure drops to
80 psi. Stop fanning to see if the governor has
placed the compressor in the loading stage (the
reading on the air gauge should increase).
· Continue fanning the brake pedal. Warning
device(s) must operate at or before 55 psi and the
trailer supply valve will close at or before 20 psi.
Continue fanning the brake pedal, the spring
parking brake should also apply at or before the
reservoir pressure drops to 20 psi.
· Rebuild reservoir air pressure at fast idle to
between 115 psi minimum and 135 psi maximum.
At 90 psi charge the trailer system by opening the
trailer supply valve.
· Set the tractor spring parking brake (if possible).
Tractor and Trailer Emergency System Check
·
·
·
·
Disconnect supply (emergency) line glad hand.
Trailer brakes should apply immediately.
There should be no air loss from trailer glad hand.
Air from the tractor should:
(a) stop immediately or,
(b) bleed down no lower than 20 psi and stop.
Note: Under no circumstances should the tractor
pressure drain below 20 psi.
· Reconnect supply (emergency) line and charge
trailer. Rebuild pressure if necessary.
74
Tractor Protection Valve Check
· Disconnect control (service) line glad hand.
· No air should escape from the tractor or trailer glad
hands.
· Release the spring parking brake.
· Make full brake application. Tractor brakes will
function normally. Trailer service brakes will not
function at all. With brake application maintained, air
will continue to escape from the control (service)
line until air pressure in the reservoirs is depleted to
no lower than 20 psi. The trailer supply and the
tractor protection valve will close and the tractor
and trailer brakes will apply. Air loss from control
(service) line will stop.
· Reconnect control (service) line.
Check for Leakage
· Rebuild reservoir air pressure at fast idle to between
115 psi minimum and 135 psi maximum, release the
spring parking brake and charge the trailer at
90 psi.
· Shut off the engine.
· Make a full brake application with the foot valve and
hold. Observe the reservoir air pressure gauge.
· The reading should not drop more than 4 psi per
minute. Lightly tap the gauge to ensure it is not
sticking. For an accurate reading brake application
should be held for at least two minutes.
· Listen for audible air leaks.
· Release full brake application.
Brake Adjustment
(A driver should be trained in brake adjustment prior
to attempting practical examination and supply the
equipment required to do a brake adjustment.)
The following are recommended steps to determine
if an S-cam brake with manual slack adjusters
requires adjustment.
· Ensure vehicle is secure and wheels blocked.
· Verify reservoirs are at full pressure.
· Shut off the engine, leaving the transmission in a
low gear or park.
· Release the spring parking brakes.
· Make a chalk mark where the push rod enters the
brake chamber, repeat this on all the brake
chambers.
· Reapply the spring parking brakes and measure
the distance from the brake chamber to the chalk
mark. Ensure the slack (push rod travel) is within
1/2 and 3/4 inches or within manufacturer’s
specifications and the angle between the slack
adjuster and push rod is 90° or as close as
practical. If not, then a brake adjustment is
required.
The following are recommended adjustment steps
for S-cam brakes with manual slack adjusters.
· Ensure the vehicle is secure and the wheels are
· Now recheck slack adjuster travel. Check that the
push rod free travel is now between 1/2 to 3/4
inches, or within manufacturer’s specifications.
Brake Test
· Reapply the spring parking brake, remove wheel
blocks.
· Gently tug against the spring parking brakes in
low gear, it should hold the vehicle.
· Release the spring parking brakes and set trailer
brakes with the hand valve.
· Gently tug against trailer brakes in low gear.
· Move vehicle ahead slowly and make a service
brake application to check brake response.
Air Over Hydraulic (Air Actuated) Brake System
The pre-trip air brake inspection should be
completed within 20 minutes. Failure to complete the
test within 20 minutes may result in the test being
discontinued.
Practical examination will be conducted in either
the metric or imperial systems of measurement,
whichever coincides with the equipment being
operated.
Vehicle Secure
· Apply the parking brake on the vehicle.
· Block the wheels; the vehicle should be on level
ground if possible.
blocked.
· Release the spring parking brakes.
· Verify reservoirs are at full pressure.
· Use a wrench to disengage the external locking
device (if so equipped) from the adjustment bolt of
the slack adjuster.
· Turn the adjustment bolt until the brake lining
contacts the brake drum. If possible, visually
check to see that the brake linings are in contact
with the brake drum.
· When turning the adjustment bolt on the slack
adjuster, the S-cam should turn in the same
direction as if a brake application were being
made.
· Back off the adjustment bolt about 1/4 to 1/2 of
a turn.
·
·
·
·
Check hydraulic fluid level in the reservoir.
Check under the hood for hydraulic fluid leaks.
Check that the compressor is secure.
Check the drive belt and pulleys for wear,
cracking, slippage and tension (if equipped).
· Drain all air reservoirs completely (wet/supply air
reservoir first). When air reservoirs are empty
close the drain valves. “For testing purposes only,
the applicant will not be required to manually
drain the reservoirs. Instead, the applicant will
verbally explain the steps that they would follow
when draining the reservoirs and then fan the
brake pedal to empty the reservoir”.
· Check wheel cylinders and lines for signs of
hydraulic fluid leaks.
75
Compressor and Warning Devices Check
Brake Test
· Start the engine and run at fast idle to build air
· Reapply the parking brake, remove wheel blocks.
· Gently tug against the parking brakes in low gear,
pressure.
· Warning device(s) must operate to 55 psi or higher.
· Ensure reservoir pressure builds from 50 to 90 psi
within three minutes.
it should hold the vehicle.
· Release the parking brakes.
· Move vehicle ahead slowly and make a service
· If equipped with spring parking brakes release them
to prevent compounding the brake pressure.
· Continue building reservoir air pressure. Check that
the governor places the compressor in the unloading stage between 115 psi minimum and 135 psi
maximum.
brake application to check brake response.
Section Summary Questions
1.
What is the maximum time permitted for the
compressor to build from 50 to 90 psi?
2.
What is the maximum air pressure loss
permitted on a full brake application with the
motor shut off?
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 the Division of Driver and Vehicle
Licencing?
· Fan the brake pedal until the pressure drops to
80 psi; stop fanning to see if the governor has
placed the compressor in the loading stage (needle
on the air gauge should start to climb).
· Continue fanning the brake pedal. Warning
device(s) must operate at or before 55 psi.
Check for Leakage
· Rebuild reservoir air pressure to between 115 psi
minimum and 135 psi maximum (if equipped release
the spring parking brakes).
· Shut off the engine.
· Make a full brake application with the foot valve
and hold. Observe the reservoir air pressure gauge.
· The reading should not drop more than 3 psi per
minute. Lightly tap the gauge to ensure it is not
sticking. For an accurate reading application should
be held for at least two minutes.
· Listen for audible air leaks.
· Release full brake application.
76
Metric Conversion Table
As some trucks have air gauges indicating pressure in
kilopascals (kPa), the following conversion chart may help.
kPa to psi
5
10
15
20
25
30
35
40
45
50
60
70
80
90
100
150
200
250
300
310
350
400
415
450
500
550
585
600
650
700
725
750
800
850
900
950
1000
1050
0.72
1.45
2.17
2.90
3.62
4.35
5.07
5.80
6.52
7.25
8.70
10.15
11.60
13.05
14.50
21.75
29.00
36.29
43.51
44.96
50.76
58.01
60.19
65.26
72.51
79.77
84.84
87.02
94.27
101.52
105.15
108.77
116.03
123.28
130.53
137.78
145.03
152.29
psi to kPa
1
2
3
4
5
6
7
8
9
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
6.89
13.78
20.68
27.57
34.47
41.36
48.26
55.15
62.05
68.94
103.42
137.89
172.36
206.84
241.31
275.78
310.26
344.73
379.20
413.68
448.15
482.62
517.10
551.57
586.04
620.52
654.99
689.47
723.94
758.41
792.89
827.36
861.83
896.31
930.78
965.25
999.73
1034.20
77
METRIC CONVERTER
mph
MAXIMUM
MAXIMUM
0
10 20
0
20
30
40 50
MAXIMUM
50
km/h
70
km/h
60
MAXIMUM
90
70 80 90 100 110
110 130 150 170
km/h
Weight
1 gram (g)
=
.035 ounce
1 kilogram (kg)
1 metric ton
=
=
2.2 pounds
1.102 ton
1 centimetre (cm)
1 metre (m)
1 metre (m)
1 kilometre (km)
=
=
=
=
.39 inch
39.3 inches or 3.27 feet
1.09 yards
.621 mile
km
=
kilometres
km/h
=
kilometres per hour
mph
=
miles per hour
Length
78
Organ Transplantation
and Donation
Transplantation
Advances in medical science now make it possible
to replace failing human organs. In fact,
replacement of a kidney, cornea, heart or liver is no
longer considered an experiment but a treatment
choice. Organs and tissues that can be effectively
transplanted are kidneys, hearts, lungs, livers,
corneas, bones, joints, skin, pancreas, bone marrow
and bowel. Therefore, one person’s decision to be
an organ donor could benefit as many as 10 people.
However, many Canadians die each year because
donor organs are not available.
Organ Donation
For many patients, transplantation offers the only
hope for leading a healthy and productive life, or, in
some cases, for life at all. By signing an organ donor
card and urging your family and friends to do the
same, you could ensure the freedom and quality of
life that you enjoy is passed on to someone else.
Your donor card confirms your wish to give the gift
of any or all tissues and organs after your death.
Although it is important to discuss your feelings
about transplantation with your family, friends or
doctor, the information on the card may help them
with decisions at a difficult time.
Age
Age is never an absolute barrier to some form of
organ donation.
If you are over 19, you may direct that any tissue or
organ may be used after your death. For persons
under 19 years, your parent or guardian should sign
the donor card also. Donor cards are available from
The Kidney Foundation or the CNIB.
For more information
For more information on transplantation and organ
donation, contact
NB Organ and Tissue Procurement Program
at (506)643-6848.
79
80
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