Airbrake Manual

Airbrake Manual
Air Brake Manual
safety
Air Brake Manual
Acknowledgments
Motor Vehicle Branch of the
Government of British Columbia
Staff Development Section
Human Resources Branch
Saskatchewan Ministry of Highways and Infrastructure
Bendix Heavy Vehicle Systems Inc.
ii
Table of contents
1. Brakes and braking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Heat – energy – traction – friction. . . . . . . . . . . . . . . . . . . . . . . . . . 3
Speed – weight – distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
How power is obtained. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Stopping distance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Section summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2. Basic system components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Wedge-type brakes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Disc brakes (rotors and pads) . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3. Basic system operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Basic air brake system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Additions to the basic system . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Reservoir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Safety valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
One-way check valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Air pressure gauge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Air governor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Relay valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Low warning switch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Stop light switch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Quick-release valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Front axle ratio valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Air dryer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Section summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4. Dual air systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Dual-circuit air system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Spring-brake chambers (emergency/park brake) . . . . . . . . . . . . . . . . . 27
Service-brake chamber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Parking-brake system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Dual-circuit system with spring parking brakes . . . . . . . . . . . . . . . . . . 30
Two-way check valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Spring brakes with modulator valve . . . . . . . . . . . . . . . . . . . . . . . . 31
Section summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
5. Tractor system/trailer towing system. . . . . . . . . . . . . . . . . . . . . .33
Tractor protection system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Trailer-supply valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
1
Manually-operated trailer-supply valves. . . . . . . . . . . . . . . . . . . . . Tractor protection valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Trailer hand-control valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Two-way check valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glad hands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bobtail proportioning relay valve. . . . . . . . . . . . . . . . . . . . . . . . . . Simple tractor-trailer system . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brake application – foot valve. . . . . . . . . . . . . . . . . . . . . . . . . . Brake application – hand valve . . . . . . . . . . . . . . . . . . . . . . . . . .
Emergency applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Service-line rupture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Supply-line rupture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Loss of supply reservoir air . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spring-brake trailer system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Section summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
35
35
36
36
37
38
40
42
44
46
48
50
52
53
6. Checking and adjusting cam-type brakes. . . . . . . . . . . . . . . . . . .
Within an inch (25 mm) of your life. . . . . . . . . . . . . . . . . . . . . . . . . Checking. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Brake adjustment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automatic or self-adjusting slack adjusters. . . . . . . . . . . . . . . . . . . Manual slack adjuster check – preferred method. . . . . . . . . . . . . . . . Brake adjustment – preferred method. . . . . . . . . . . . . . . . . . . . . . Brake adjustment – alternate method. . . . . . . . . . . . . . . . . . . . . . Service tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stroke vs. force. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Steep downgrade. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Pre-trip procedure for air single unit. . . . . . . . . . . . . . . . . . . . . . . . Pre-trip procedure for air combination unit . . . . . . . . . . . . . . . . . . . . .
Section summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
54
55
55
56
56
56
56
57
58
58
59
60
62
7. Glossary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63
8. Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66
9. Air brake manual summary. . . . . . . . . . . . . . . . . . . . . . . . . . . .68
10. Conversion charts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
2
1. Brakes and braking
Heat – energy – traction – friction
To move a vehicle,
­­
an internal combustion engine must convert its heat energy to
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 a vehicle is the amount of traction its tires have on the road
surface.
Traction is the ability of a tire to grip the road surface on which it rolls. The vehicle’s
acceleration rate depends on the power the engine develops and the amount of
traction the tires have on the road surface.
Friction is the force which resists movement between two surfaces in contact with
each other. To stop a vehicle, 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. 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 heat by passing it off into the atmosphere.
The amount of heat the brake drums can absorb depends on the metal thickness
of which they are made. When enough friction is created between brake linings
and drums, the wheels stop turning. The final factor that stops a vehicle is not the
brakes, but the traction between tires and 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. Not only that, the vehicle
might have to be stopped in an emergency, in as little as six seconds (just 1/10 of
the time it took to reach 100 km/h).
To stop a vehicle in 1/10 the time it takes to accelerate requires stopping power of
10 times the acceleration power – equivalent to 2,000 horsepower.
Figure 1. Horsepower.
3
If the vehicle had six wheels, each wheel would have to provide 1/6 of the braking
power. If one or two 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 build-up of heat greater than the brake drums could absorb
and dissipate. Too much heat would result in brake damage and possible failure.
Most brake linings operate best around 250°C and should not exceed 425°C
(Fig. 2). It’s important to understand that the power needed to stop generates heat
which could ruin the brakes.
250°C
NORMAL
425°C
MAXIMUM
1100°C
PANIC!
Figure 2. Brake lining temperature.
Speed – weight – distance
The distance required to stop a vehicle depends on its speed and weight in
addition to the factors of energy, heat and friction. The brake power required to
stop a vehicle varies directly with its weight and the “square” of its speed. For
example, if weight is doubled, stopping power must be doubled to stop in the
same distance. If speed is doubled, stopping power must be increased four times
to stop in the same distance. When weight and speed are both doubled, stopping
power must be increased eight times to stop in the same distance.
Example: A vehicle carrying a load of 14,000 kg down a grade at 16 km/h is
brought to a stop in a distance of 30 metres by normal brake application. If the
same vehicle carried 28,000 kg down the same grade at 32 km/h, it would require
eight times the braking power to stop the vehicle in 30 metres. This would be
more braking power than the brakes could provide. No vehicle has enough braking
power when it exceeds its limitations.
4
How power is obtained
A. Mechanically
Braking systems use devices to gain a mechanical advantage. The most common
device for this purpose is leverage (Fig. 3).
A lever is placed on a pivot called the fulcrum. If the distance from A to C is four
metres, and from C to B one metre, the ratio is four to one (4:1). Power is multiplied
by the leverage principle. If a 100 kg downward force is applied at point A, then
upward force at point B is 400 kg. This is the result of the mechanical advantage
of leverage.
100 kg
C
4
1
A
B
400 kg
Figure 3. Simple lever.
B. Use of air
Force can also be multiplied by the use of air to gain a further mechanical advantage.
Everyone has felt the power of air on a windy day. Air can be compressed into a
much smaller space than it normally occupies. For instance, air is 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.
Applied force
Delivered force
A
B
C
A
A
C
B
B
C
B
Figure 4. Various levers. Compare points A, C, B
to the previous lever diagram (Fig. 3).
5
If a constant supply of compressed air is directed through a pipe that is one-inch
square, and if a one-square-inch plug was placed in the pipe, the compressed air
would push against the plug. Holding a scale against the plug would register how
many pounds of force were being exerted by the air against the plug.
If the scale registered 10 lb., for example, then it could be said the force was 10 lb. on the
one-square-inch surface of the plug (Fig. 5). This would be 10 lb. per square inch (psi).
The more the air in the supply tank has been compressed, the greater the force
that would be exerted on the face of the plug.
1 Inch
10 psi
Figure 5. Pounds per square inch (psi).
C. Leverage and air pressure
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 (Fig. 6), 3,600 lb. of force is produced (120 x 30). Apply this
force to a push rod to move a six-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 between 80 and 100 foot pounds of torque to tighten
the wheel on a car. This comparison illustrates the power obtained from using
mechanical leverage and air pressure combined.
120 psi
6"
30 Sq. In.
1"
Figure 6. Air pressure combined with leverage.
6
Stopping distance
In addition to the factors mentioned on page 3, a driver must understand what is
meant by the term stopping distance. Stopping distance consists of three factors:
Driver’s reaction time + Brake lag + Braking distance.
Reaction time:
The time it takes from the moment a hazard is recog­nized to the time the brake
pedal is applied, approximately 3/4 of a second.
Brake lag:
The time air takes to travel through a properly main­tained air brake system, about 4/10
of a second.
Braking distance:
The actual distance a vehicle travels after the brake is applied until the vehicle
stops. This distance depends on the ability of the lining to produce friction, the
brake drums to dissipate heat and the tires to grip the road.
Professional drivers never take brakes for granted. The braking system must be
tested and adjustment checked before placing a vehicle into service. Pro­fessionals
understand the braking system, realize its capabilities and limitations, and learn to
use it to their 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 heat becomes too great, braking effectiveness will be lost. The
heavier the load and the faster the speed, the greater the power needed to stop.
Stopping distance is also affected by the driver’s reaction time, brake lag and
braking distance (Fig. 7). The professional driver is well aware that the vehicle, even
with properly adjusted brakes, will not stop as quickly as a passenger vehicle.
Double vehicle weight – Double your stopping distance
Double vehicle speed – Quadruple your stopping distance
Figure 7. Stopping distance.
7
Section summary
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 heat generated by the brakes dissipated?
4. If one set of brake shoes is poorly adjusted, what effect could it have on the
remaining set 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 vehicle speed is doubled, how many times must stopping power be
increased to be able to stop in the same distance?
8. If vehicle weight and speed are both doubled, how many times must the
stopping power be increased to be able to stop in 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, what is the total
pounds of force that could be exerted?
12. What is meant by the following terms: reaction time, brake lag, braking
distance and stopping distance?
8
2. Basic system components
The five main components of an “elementary” air brake system and their
purposes are:
1.Compressor:
to build up and maintain air pressure
2. Reservoirs:
to store the compressed air
3. Foot valve:
to draw compressed air from reservoirs when it is needed for braking
4. Brake chambers:
to transfer the force of compressed air to mechanical linkages
5. Brake shoes and drums or brake rotors and pads:
to create the friction needed to stop the vehicle
1.Compressor
The function of the air compressor (Fig. 8) is to build up and maintain air pressure
required to operate air brakes and air-powered accessories.
Air compressors are either gear driven directly from the engine or belt driven.
Although most compressors use the truck’s lubrication and cooling systems, some
are self-lubricated and some are air cooled. Self-lubricated compressors must
have their oil checked and changed at regular intervals.
The compressor’s intake system draws air from either its own air filter or from the
engine’s intake system.
Compressors that have their own filtration system must be serviced on a regular
basis.
All compressors run continuously while the engine is running, but air compression
is controlled and limited by a governor which “loads” or “unloads” the compressor.
In the loaded stage, air is pumped into reservoirs. In the unloaded stage (with two
cylinder compressors), the compressor pumps air back and forth between the two
cylinders without supplying the reservoirs.
9
Exhaust
valve
Inlet
valve
Piston
and ring
assembly
Crankshaft
Figure 8. Air compressor.
The governor must take the compressor out of its pumping stage (unload/cut-out)
when system air pressure reaches 115-135 psi (793-931 kPa), and also put it
back into the pumping stage at a minimum of 80 psi (552 kPa).
2.Reservoirs
Reservoirs are pressure-rated tanks, which hold a supply of compressed air until
required for braking or operating auxiliary air systems. They must store a sufficient
volume of air to allow several brake applications if the engine stops or the
compressor fails.
The maximum air pressure available for brake applications depends on how much
air is in the reservoir. A driver is not able to make a higher pressure brake
application than there is air pressure in the reservoir.
Each reservoir is equipped with a drain valve called a draincock (Fig. 9). Fully
opening the draincock allows reservoirs to be drained of moisture and other
contaminants that build up in the system. All reservoirs must be completely
drained once a day when in use.
Figure 9. Typical reservoir drain valves.
10
3.Foot valve (application or treadle valve)
This foot-operated valve (Fig. 10) applies air to operate the brakes. The amount of
air delivered to the brakes is regu­lated by the driver according to the distance the
treadle or brake pedal is depressed. Releasing it exhausts air in the service brakes
through its exhaust port.
These valves are made in overhead styles with a foot pedal hanging down, or a
floor-mounted version with a foot treadle.
Treadle
Delivery
Supply
Exhaust port
Figure 10. Dual-circuit foot valve.
4.Service-brake chambers (brake pots)
Service-brake chambers (Fig. 11) convert compressed air pressure energy into
mechanical force and move­ment, which apply the vehicle’s brakes.
When the driver applies pressure to the foot valve, air pressure enters the pressure
side of the brake chamber through the inlet port and forces against the diaphragm,
which moves the push rod assembly forward. When air pressure is released from
the service-brake chamber, the return spring returns the diaphragm and push rod
to their released positions.
11
Inlet port
Return spring
Mounting bolts
Diaphragm
Push rod assembly
Yoke
Inlet port
Lock nut
Service-brake chamber released
Return spring
Inlet port
Mounting bolts
Push rod assembly
Diaphragm
Yoke
Inlet port
Lock nut
Service-brake chamber applied
Figure 11. Clamp-ring type service brake chamber.
5.Brake shoes and drums
Figure 12 illustrates the common S-cam brake assembly used on truck and trailer
axles. Front brake assemblies have the brake chamber and slack adjuster mounted
on the backing plate because the steering action of the front axle would otherwise
interfere.
The diagram shows the brakes in the applied position. The S-cam is rotated so the
high points have acted against the cam rollers and forced the brake shoes against
the drum.
When the brakes are released, the brake cam shaft returns the brake cam to the
normal position. The cam rollers roll down into the crook of the S-cam as the brake
shoe return spring pulls the shoes away from the drum.
Brake lining material is attached to the face of the shoes. Lining material is selected
according to the type of service the brakes are subjected to. Linings must give
consistent braking output with minimum fade at high temperatures.
Brake shoes generate heat through friction with the brake drum surface. Drum
thickness determines the amount of heat that can be absorbed and dissipated
to the atmosphere. Thin or distorted drums, weak return springs, improper
linings, poor adjustment, or grease or dirt on the lining, will all result in erratic,
unpredictable and potentially dangerous brake performance.
12
Brake drum
Slack adjuster
Adjusting
nut
Brake cam shaft
Brake cam
Cam roller
Axle
Return
spring
Brake shoe
Figure 12. S-cam brake assembly.
Wedge-type brakes
Air chamber push rod action forces a wedge-shaped push rod between the brake
shoe rollers. This forces the brake shoe lining against the brake drum. Each wheel
may be equipped with one or two chambers, depending on vehicle size and style.
These brakes may be equipped with a self-adjusting mechanism or a manual star
wheel adjuster. The star wheel adjustment is made with the vehicle jacked up,
to ensure that the brake linings do not drag. Manual adjust­ment of wedge-type
brakes is usually a job for a mechanic.
Disc brakes (rotors and pads)
Some trucks are now equipped with disc brakes. Disk brakes can be either
non-adjustable sealed units or adjustable. If your truck is so equipped, consult the
owner’s manual for information on how to keep the brakes properly adjusted.
Rotor
Brake
Chamber
Brake Pad Friction
Material
Figure 13. Air disc brake.
13
3. Basic system operation
Basic air brake system
Air is pumped by the compressor to the reservoir. When air pressure reaches 115
to 135 psi (793 to 931 kPa), the governor places the compressor into its unloaded
stage. At this stage the air system is fully charged (Fig. 14).
4
4
3
1
4
4
1. Compressor
2. Reservoir
3. Foot valve
4. Brake chambers
2
Figure 14. Basic air brake system.
When the brakes are applied, air is delivered through the foot valve to the servicebrake chambers (Fig. 15). Air pushes against each service-brake diaphragm
causing the push rod to move the slack adjuster (see page 55). The slack adjuster
rotates the brake cam, which forces the shoes against the brake drum.
When the driver releases the foot valve, air in the brake chambers is exhausted
through the foot valve, which releases the brakes.
When reservoir air pressure drops, the governor puts the compressor back into the
pumping stage to keep adequate air pressure available for future brake applications.
4
3
4
1
4
4
1. Compressor
2. Reservoir
3. Foot valve
4. Brake chambers
2
Figure 15. Brake application.
14
Additions to the basic system
Several additions (Fig. 16) can be made to the basic system to improve it:
• service reservoir
• air pressure gauge
• low warning switch
• relay valve
• front-brake limiting valve or automatic front brake ratio valve
• one-way check valve
• stop light switch
Reservoir
Figure 16 shows that an additional air reservoir has been added. Since the first
reservoir is closest to the com­pressor it is now called the supply reser­voir. It is
sometimes called the wet reservoir because most of the water and oil from the
compressor gathers here.
The second reservoir is called the service reservoir. Air is drawn from this reservoir
to operate the brakes.
8
12
12
10
13
13
Con
trol li
ne
11
7
9
1
6
12
13
12
3
5
2
13
4
1. Compressor
2. Supply (wet) reservoir
3. Safety valve
4. One-way check valve
5. Service reservoir
6. Low warning switch
7. Air pressure gauge
18. Foot valve
19. Automatic front brake
ratio valve
10. Stop light switch
11. Relay valve
12. Brake chambers
13. Slack adjusters
Figure 16. Basic system plus additions.
15
Safety valve
The supply reservoir is protected from being over-pressurized and bursting by a
safety valve (Fig.17). This valve is pre-set (usually at 150 psi [1,034 kPa]) and will
blow off excess pressure. Once pressure is lowered, the safety valve will re-seal
until an over-pressurized condition exists again. If a safety valve blows off excess
pressure, this indicates a problem with the governor. The problem should be dealt
with immediately by a qualified person.
Exhaust port Spring
Ball seat
Adjusting nut
Valve stem
Inlet from reservoir
Lock nut
Figure 17. Safety valves.
One-way check valve
In case the air compressor fails or a leak develops in the supply reservoir, a
one-way check valve (Fig. 18) is installed between the supply and service
reservoirs to keep air from bleeding back. The valve is spring loaded. Pressure at
the inlet side overcomes spring pressure and lifts a check 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 spring pressure, the check device seals,
preventing air from flowing back through the valve.
Out
In
Figure 18. One-way check valve.
16
Air pressure gauge
An air pressure gauge (Fig. 19) is installed in the dash (plumbed in after the
service reservoir) so the driver will know the amount of air pressure available for
braking.
100
80
600
60
120
140
800
400
1000
40
200
160
1200
180
kPa
psi
20
Air pressure
200
Figure 19. Air pressure gauge.
Air governor
The governor (Fig. 20), which is usually compressor mounted, operates in conjunction
with the compressor and maintains reservoir air pressure between a predetermined
maximum and minimum pressure.
• cut-out pressure 115 to 135 psi (793 to 931 kPa) maximum
• cut-in pressure 80 psi (552 kPa) minimum
The governor will normally cut in 20-25 psi below the cut-out pressure, however
the minimum cut-in pressure is 80 psi.
Exhaust port
Mounting holes
Unloader ports (3)
Reservoir ports (3)
Figure 20. Air governor.
17
Relay valve
On long wheelbase trucks and tractors and on trailers, the distance from the brake
chambers to the foot valve is too far to cause immediate application of the brake
when the foot valve is depressed. This is called brake lag. To correct this situation,
a relay valve (Fig. 21) is installed near the rear brake chambers. A large diameter
pipe is connected between the service reservoir and relay valve. The air line from
the foot valve to the relay valve now becomes a control line that signals to the relay
valve the amount of air to be drawn from the service reservoir for faster application
of the brakes. A quick-release valve is built in for faster release of the brakes.
Supply port
(reservoir mount type)
Service
port
Supply
port
Delivery
ports (4)
Exhaust
Relay piston
Return spring
R-6 insert
Figure 21. Relay valve.
Low warning switch
A low warning switch is installed after the supply reservoir to alert the driver
when air pressure drops below a safe level (about 55 psi [379 kPa]). The switch
activates either (or a combination of) a buzzer, warning light or a wig-wag (a “flag”
that drops into the driver’s view). If the low warning system activates, the driver
must stop and determine the cause. The warning light must be operational. The
buzzer/wig-wag are optional.
18
Stop light switch
The stop light switch (Fig. 22) is an air-signaled electrical switch which is turned
on any time a brake application is made. The switch is usually connected to a
double check valve and can be plumbed anywhere in the application side of the
circuit. In a tractor system it is usually plumbed into the double check valve that is
matched with the tractor protection valve.
Terminal
Cover
Terminal
connector
Contacts
Spring
Plunger
Washer
Diaphragm
Body
Figure 22. Stop light switch.
Quick-release valve
The function of a quick-release valve (Fig. 23) is to rapidly exhaust air from the
controlled device. It is normally located adjacent to the controlled device, rather
than requiring exhaust air to return and exhaust through the control valve. This
decreases release time.
Figure 23. Quick-release valve.
19
Front axle ratio valve
Designed for use on dual-air system vehicles, the ratio valve (Fig. 24) is installed in
the front axle delivery line. During normal brake applications, this valve automatically
reduces application pressure to the front axle brakes. As brake application
pressure increases, the percentage of reduction is decreased until about 60 psi
(413 kPa) (depending on valve design) when full pilot pressure is delivered. The
valve is available with several different “hold-off” pressures, which prevent the front
brakes from operating until this “hold-off” pressure is exceeded.
Figure 24. Front axle ratio valve.
Note: Older trucks may be equipped with a front wheel limiting valve controlled by
a switch on the dash. When activated, this valve will reduce application pressure
on the steering axle brakes by 50%.
Air dryer
The air dryer (Fig. 25) is a desiccant-type in-line filtration system that removes most
liquid and water vapour from compressor discharge air before it reaches the air
brake reservoirs. This results in only clean, dry air being supplied to the air brake
system, aiding in the prevention of air-line freeze-ups.
20
Figure 25. Air dryer.
Air dryers utilize a replaceable desiccant material that has the ability to strip water
vapour from moisture laden air. The desiccant material is regenerative, in that its
absorptive properties are renewed each time the compressor is reloaded.
The air dryer end cover is equipped with an automatic drain valve, controlled by the
air-system governor, and is also equipped with an integral heating element.
Air dryers do not remove all the moisture. The reservoirs still need to be drained
daily when in use.
Section summary
1. How can the driver tell how much air pressure there is in the main reservoir?
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. How is the reservoir protected from over-pressurization?
6. At what pressure will the low pressure warning device activate?
7. How is “brake lag” to rear wheels minimized?
21
4. Dual air systems
Note: All piping diagrams are used to illustrate basic dual circuit principles only,
and are not to be interpreted as regulations for, or specifications of, dual air-brake
systems.
Virtually all heavy-duty vehicles on the road today are using a dual-circuit air system
(Fig. 26). The system has been developed to prevent total brake failures and give
the driver more control by allowing the truck to be brought to a stop in a safe
location (Fig. 27). At first glance, the dual system might seem complicated, but
if you understand the basic air system described so far, and if the dual system is
separated into its 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 two-axle vehicle,
one circuit operates from the primary reservoir and the other circuit operates from
the secondary reservoir.
If one circuit has a failure, the other circuit is isolated and will continue to operate.
Under normal operating conditions the primary reservoir operates the rear service
brakes and the secondary reservoir operates the front service brakes.
Low pressure
switch
Air
dryer
Safety
valve
Secondary
reservoir
One-way check
valve
Supply
reservoir
One-way check
valve
Low pressure
switch
Gauge
Primary
reservoir
Governor
Compressor
Supply circuit
Primary circuit
Figure 26. Simple dual circuit.
22
Secondary circuit
Gauge
Low pressure
switch
Air
dryer
One-way check
valve
Safety
valve
Supply
reservoir
Gauge
Secondary
reservoir
Low pressure
switch
Gauge
One-way check
valve
Governor
Primary
reservoir
Compressor
Supply circuit failure
Low pressure
switch
Air
dryer
One-way check
valve
Safety
valve
Supply
reservoir
Secondary
reservoir
Low pressure
switch
Gauge
One-way check
valve
Governor
Gauge
Primary
reservoir
Compressor
Secondary circuit failure
Low pressure
switch
Air
dryer
One-way check
valve
Safety
valve
Supply
reservoir
Governor
Gauge
Secondary
reservoir
Low pressure
switch
Gauge
One-way check
valve
Primary
reservoir
Compressor
Primary circuit failure
Supply circuit
Primary circuit
Figure 27. Simple dual-circuit failures.
23
Secondary circuit
Dual-circuit air system
In Figure 28, air is pumped by the compressor to the supply reservoir, which is
protected from over-pressurization by a safety valve. Pressurized air moves from
the supply reservoir to the primary reservoir (green) and the secondary reservoir
(red) through one-way check valves. At this point, the dual circuits start. Air from
the primary reservoir is directed to the foot valve. Air is also directed from the
secondary reservoir to the foot valve. The foot valve is divided into two sections
(two foot valves in one). One section of this dual foot valve controls the primary
circuit and the other section controls the secondary circuit.
Brake
chambers
Foot
valve
Ratio valve
Air
dryer
Safety
valve
Low
pressure
switch
Supply
reservoir
Governor
Relay
valve
Gauge
Low pressure
switch
One-way check
Secondary
valve
reservoir
One-way check
valve
Gauge
Primary
reservoir
Compressor
Supply circuit
Primary circuit
Secondary circuit
Figure 28. Simple dual circuit with brakes released.
24
When a brake application is made (Fig. 29), air is drawn from the primary reservoir
(green) through the foot valve and is passed on to the relay valve, which delivers
air from the primary reservoir to the rear brake chambers. At the same time, air is
also drawn from the secondary reservoir (red), passes through the foot valve and is
passed on to the front brake chambers.
If there is an air loss in either circuit, the other circuit will continue to operate
independently (Fig. 30 and Fig. 31). Unless air is lost in both circuits, the vehicle will
continue to have braking ability. The primary and secondary circuits are equipped
with low-pressure warning devices and pressure gauges.
Brake
chambers
Foot
valve
Ratio valve
Air
dryer
Safety
valve
Supply
reservoir
Governor
Relay
valve
Gauge
Low pressure
switch
One-way check
valve
Secondary
reservoir
One-way check
valve
Low
pressure
switch
Gauge
Primary
reservoir
Compressor
Supply circuit
Primary circuit
Secondary circuit
Figure 29. Simple dual circuit with brakes applied.
25
Brake
chambers
Foot
valve
Ratio valve
Air
dryer
Safety
valve
Low
pressure
switch
Gauge
Supply
reservoir
Governor
Relay
valve
Low pressure Gauge
switch
One-way check
valve
Secondary
reservoir
One-way check
valve
Primary
reservoir
Compressor
Supply circuit
Primary circuit
Secondary circuit
Figure 30. Secondary circuit failure with brakes applied.
Brake
chambers
Foot
valve
Ratio valve
Air
dryer
Safety
valve
Governor
Compressor
Supply circuit
Relay
valve
Low pressureGauge
switch
One-way check
valve
Secondary
reservoir
Low
pressure
switch
Supply
reservoir
One-way check
valve
Gauge
Primary
reservoir
Primary circuit
Secondary circuit
Figure 31. Primary circuit failure with brakes applied.
26
Spring-brake chambers (emergency/park brake)
A spring-brake chamber functions as a service-brake chamber, an emergency
brake in case of air-pressure loss somewhere in the system, and as a reliable
spring-applied parking brake (Fig. 33). Spring brakes are installed in the same
manner as service brakes and are always installed on the front tandem axle. Spring
brakes are often installed on both rear axles in a tandem-axle unit. They are a reliable
parking brake because they are held on by spring pressure and require no air.
Spring brakes consist of two separate air chambers. The front chamber is essentially
a service-brake chamber, and is used to perform the service-brake function. The
rear chamber houses a large, powerful compression spring and diaphragm and
performs emergency and parking functions. It is sometimes called a “piggyback.”
CAUTION: Never disassemble a spring brake. Serious injury may result. All
discarded spring-brake chambers must be dis­assembled and disposed of by
a trained professional.
Service-brake chamber
The service-brake chamber applies the brake by air pressure and releases it by
spring pressure (just like a single service-brake chamber).
Using an opposite action, the spring-brake chamber applies the spring brake by
spring pressure and releases it by air pressure. In the event of air-pressure loss
(an emergency or an intentional exhausting of air by the driver, e.g., while parking),
the power spring will push the diaphragm and push rod down and apply the brake.
During normal operation, air pressure keeps the power spring compressed and
allows the service brake to operate normally.
If air pressure cannot be restored and it is necessary to move the vehicle, the
power spring can be compressed manually by the use of a wind-off bolt.
Parking-brake system
Installation of parking brakes and piping arrangements into a vehicle air brake
system will vary, depending on the vehicle make.
Control valves will vary, depending on the manufacturer and type of piping
arrangements.
27
The type of spring-loaded valve shown (Fig. 32) requires that the driver push the
button to release the parking brakes. If the air pressure in the system falls below
approximately 60 psi, the spring brakes may begin to drag and if it falls between
20-45 psi (138-310 kPa), will fully apply. On many vehicles the parking brake
control valve on the dash will close at about 20 psi, however some valves may
never close. The important thing is that the spring brakes are fully applied before
the air is depleted. Always ensure the spring brakes have been fully applied.
Similar types of spring-loaded valves require the driver to pull the button out to
release the parking brakes.
Note: On some newer models the park brake button will not pop out automatically.
However, the brakes will still apply.
Note: There is a toggle control valve in use that does not have an automatic brake
application feature. The park brakes will gradually apply as the air pressure is
depleted, however, the control valve will not move. When air pressure is restored,
the park brakes will release if the toggle valve is not manually moved to the park
brake “on” position.
pu
sh
se
PARKING
BRAKE
t o r ele a
Figure 32. Park-brake control valve.
CAUTION: Compounding the brakes happens when a service brake application
is made with the park brake still applied. This can result in damaged brake
components and possibly brake failure. To avoid compounding, the park brake
should be released before a foot brake application is made.
Note: An anti-compound line (see Fig. 34) is sometimes installed between the
delivery side of the primary circuit relay valve and the control side of the relay valve
operating the spring brakes. When a brake application is made, the relay valve
operating the spring brakes gets a signal from the service brake to release the
spring brakes with the same amount of pressure applied to the service brakes.
This prevents service-brake and spring-brake pressure from compounding on the
brake linkages.
28
System charged –
normal running condition
C
With air pressure of 70 psi (483 kPa)
or greater acting upon the emergency
diaphragm (A) and piston (B) in the
spring hold-off cavity, the spring (C) is
fully compressed and the piston (B) is
held in the released position. This does
not affect the service diaphragm (D) or
service push plate and rod (E).
C
C
A
A
During a controlled service brake
application, air pressure enters the
service port and acts
upon the service
Spring-brake
hold-off pressure
diaphragm (D), which
forces the service
push plate and rod (E) forward, applying
force to the slack adjuster. The slack
adjuster rotates the camshaft and
applies the brakes. The emergency
spring is held in the
compressed
Spring-brake
hold-off pressure
position by air pressure
in the spring
hold-off cavity.
E
D
B
D
B
E
F
E
F
C
C
A
B
D
A
B
D
B
D
E
E
F
F
C
A
E
F
C
A
D
B
C
C
A
B
A
B
ED
D
E
E
F
F
C
A
D
B
Service
air pressure
Spring-brake
hold-off pressure
Service application
Spring-brake
hold-off pressure
D
B
F
Park and emergency application
When the driver operates the park
control valve, air is exhausted from
the spring hold-off cavity. The spring
(C) is now allowed to extend, forcing
the piston (B) and the diaphragm (A)
forward. The piston (B) forces the
service diaphragm (D) and service push
plate and rod (E) forward compressing
the return spring (F) and applying the
brakes. To release the park application,
the park control valve is placed in the
“release” position, releasing the brakes
as described under “System charged –
normal running condition.”
A
C
A
Service
air pressure
Service
air pressure
Service
air pressure
Atmosphere
pressure
C
A
B
B
D
D
E
E
Atmosphere
pressure
Atmosphere
pressure
Figure 33. Spring brakes.
Atmosphere
pressure
29
E
Dual-circuit system with spring parking brakes
When spring brakes are added to a dual-circuit system, the same type of dash
control valve discussed pre­viously is used (Fig. 34). Blended air is used to supply
the control valve. Blended air is taken from the primary and secondary circuits
through a two-way check valve.
Brake
chambers
Park
brake
control
Foot
valve
Ratio valve
Two-way
check
valve
Air
dryer
Gauge
Low
pressure
One-way check switch
valve
Secondary
reservoir
Safety
valve
Supply
reservoir
Governor
One-way check
valve
Relay
valve
Relay
valve
Anti-compound
line
Low pressure
switch
Gauge
Primary
reservoir
Compressor
Supply circuit
Primary circuit
Secondary circuit
Spring-brake circuit
Figure 34. Dual-circuit system with spring parking brakes. Relay valve installed in
spring-brake circuit to quickly apply and release spring brakes.
Two-way check valve
This valve (Fig. 35) allows air to be directed to one delivery pipe from either of two
sources. A two-way check valve allows the source applying the higher pressure to
shift the shuttle so that the higher pressure will be directed to the delivery port.
With this piping arrangement, the vehicle can have a failure in either circuit without
the spring brakes applying auto­matically. Unless air is lost in both circuits, the
spring brakes will not apply.
In Figure 35 the primary circuit has a higher air pressure than the secondary
circuit. The shuttle has blocked off the secondary port and the spring brakes are
held off by primary air pressure. This also works in the opposite way.
30
·
Secondary
circuit
Primary
circuit
Shuttle
·
To spring brakes
Figure 35. Two-way check valve.
Spring brakes with modulator valve
Spring-type brakes in this system serve two purposes: first, as a parking brake and
second, as an emergency system.
If a failure occurs in the primary circuit and a brake application is made, control air from
the second­ary side of the foot valve is directed to a spring-brake modulator (Fig. 36).
As there is no primary supply air to maintain balance in the modulator valve (due
to the primary circuit failure), the modulator valve then exhausts air pressure from
the spring-brake circuit. The amount of air released is equal to the amount of air
applied by the foot valve. Release of air in the spring-brake circuit causes the drive
axle to brake using spring-brake pressure. When the brake is released, supply air
from the secondary circuit returns the spring brakes to an off position.
Brake applications can be repeated until all the air from the secondary circuit is
lost, but as air pressure drops below 70 psi (483 kPa), the spring brakes won’t
return to full off position – in fact, they will start to drag. At about 20 psi (138 kPa),
the spring-brake control valve on the dash exhausts the remaining air in the springbrake circuit, and the spring 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 use the wind-off bolts to compress the power spring. This process is
called caging the brakes.
31
Brake
chambers
Park
brake
control
Modulator
valve
Foot
valve
Ratio valve
Two-way
check
valve
Air
dryer
Gauge
Low pressure
switch
One-way check
valve
Secondary
reservoir
Safety
valve
Supply
reservoir
One-way check
valve
Governor
Relay
valve
Anti-compound
line
Relay
valve
Low pressure
switch
Gauge
Primary
reservoir
Compressor
Supply circuit
Primary circuit
Secondary circuit
Spring-brake circuit
Figure 36. Spring brakes with modular valve – primary circuit failure.
Section summary
1. What is the basic principle of the dual-circuit system?
2. What valve is used to protect the primary circuit from the secondary circuit?
3. In a dual-circuit system, will the vehicle continue to have braking ability if one
circuit fails?
4. What is meant by “compounding” the brakes?
5. Why are spring brakes a reliable type of parking brake?
6. How are parking brakes held in the released position?
7. What is the reason for releasing the parking brakes before making a full brake
application test?
8. What is the danger of disassembling a parking-brake unit?
9. Name two functions of the spring brakes in a dual-circuit system.
10. Describe the functions of the spring-brake modulator valve.
11. What is blended air?
32
5. Tractor system/trailer towing system
To change a two- or three-axle unit into a tractor, a tractor system must be added.
It consists of the following components:
Tractor protection system
The trailer-supply valve and tractor protection valve make up the tractor protection
system. This system prevents air loss from the tractor when not hooked to a trailer
or if a trailer breaks away. The minimum pressure at which the tractor protection
system must be activated is 60 to 20 psi (413 to 138 kPa).
Trailer-supply valve
This valve is essentially another dash-mounted control valve (Fig. 37). It has two
functions:
1. It controls the tractor protection valve. The tractor protection valve will not
operate if the trailer-supply valve is closed.
2. It serves as a link between the tractor and the trailer parking-brake systems by
supplying air to the trailer reservoirs, through the supply line.
Air is supplied to the trailer-supply valve by a double-check valve that is connected
to both the primary and secondary circuits. The double-check valve only takes air
from the highest pressure circuit, which prevents loss of air from a failed circuit.
h to suppl
us
y
p
This valve (usually a red octagonal button) is mounted in the cab of the vehicle,
easily accessible to the driver. The driver opens the valve by pushing or pulling the
button, depending on the type used.
pu
ll t
o evacua
Figure 37. Trailer-supply valve.
33
te
TRAILER
SUPPLY
not for parking
Opening the valve permits main reservoir pressure to flow through the valve. This
pressure is piped to the tractor protection valve and the supply-line glad hand.
The spring-loaded valve is held in the open position when sufficient air pressure
is reached. If pressure drops to between 60 and 20 psi (413 to 138 kPa), some
valves will shut automatically by spring pressure, opening the exhaust port. On
some vehicles the button may not pop out, however the spring brakes will apply.
Always ensure the spring brakes have been fully applied. The driver can close the
valve manually to uncover the exhaust port.
Note: The trailer-supply valve has also been referred to as the emergency valve.
Manually-operated trailer-supply valves
Some vehicles are 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 important difference is that this trailer-supply
valve must be shifted to the EMERGENCY position manually.
Charging the trailer system:
The driver places the trailer-supply valve in the NORMAL position and reservoir air
will be directed to the tractor protection valve and supply-line glad hand.
Trailer breakaway:
Rapid loss of air pressure in the supply line will cause the trailer brakes to dynamite.
“Dynamiting” is an emergency applica­tion that occurs when the emergency part of
the valve directs trailer reservoir pressure to the trailer brakes. To prevent air loss
from the tractor, the driver must shift the trailer-supply valve to the EMERGENCY
position, otherwise the tractor air pressure will bleed down and hold at between
60 and 40 psi (413 to 275 kPa).
Figure 38. Tractor protection valve.
34
Tractor protection valve
A tractor protection valve (Fig. 38) is usually mounted on the cab or chassis of the
tractor.
When the trailer-supply valve is open, air passes through the bottom of the tractor
protection valve and charges the trailer through the supply line (also called the
emergency line).
When the pressure in the supply line reaches 45 psi, the service line port of the
tractor protection valve opens. This allows application air pressure to travel down
the service line to the trailer when a brake application is made.
Note:
• The supply line always contains the same air pressure as is in the highest-pressure
circuit (provided the trailer-supply valve is open).
• The service line only contains air pressure when a brake application is made and
the trailer-supply valve is open.
• When you are not hooked to a trailer, the trailer-supply valve is closed and there
will be no air to the tractor protection valve. Spring pressure closes the service
line port. This action protects the application air pressure in the truck.
• On a trailer breakaway, air will rush out of the supply line until the trailer-supply
valve automatically closes (automatic type). This prevents any more loss of air
from the tractor.
Trailer hand-control valve
The hand valve (Fig. 39) is added so that the driver can apply the trailer’s brakes
independent of the tractor.
The hand valve is typically supplied from primary and secondary circuits and
plumbed to a double-check valve (which is also fed from the foot valve). The
double-check valve isolates either the foot valve or the hand valve, depending on
which one has the highest application pressure.
Note: Some power units are now being manufactured without a hand valve.
Figure 39. Trailer hand-control valve.
35
Independent operation of the trailer brakes has two common uses:
• To couple or uncouple the trailer.
• In the event that the tractor goes into a skid, gentle brake applications using the
hand valve may be of some use in trying to straighten out the unit (never apply
the hand valve if the trailer goes into a skid).
CAUTION: The hand valve is not to be used as a parking brake.
Two-way check valve
The two-way check valve (Fig. 35) allows control of the trailer brake by use of
the hand valve or foot valve. This valve will permit air to flow from the source that
is supplying the higher application pressure. Two-way check valves are installed
between the hand valve and the tractor protection valve, and between the foot
valve and the tractor protection valve. Two-way check valves can permit a higher
brake application to the trailer than the truck.
Glad hands
This term refers to the coupling device used to connect the service and supply
lines of the trailer to the truck or tractor. These couplers have a snap-lock position
and a rubber seal that prevents air from escaping.
Before connection is made, couplers should be clean and free of dirt and grit.
When connecting the glad hands, start with the two seals together and the
couplers at a 90-degree angle to each other. A quick, downward snap will join
and lock the couplers. Vehicles equipped with “dead-end” couplers should have
protection plates in use whenever the vehicle is used without a trailer. This will
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 service
line can be locked to the glad hand of the supply line to keep water and dirt from
entering the unused lines. The cleaner the air supply is kept, the less chance of
brake problems.
Glad hands and lines should also be secured to prevent the line from bouncing off
the vehicle. This could seriously damage the couplers.
36
Rubber seal
(“O” ring)
Flexible air
line from
tractor
Metal
“glad hand”
Truck line
Trailer line
“O” ring
Representation of a coupler
Figure 40. Glad hands.
Bobtail proportioning relay valve
Some trucks are equipped with a bobtail proportioning relay valve (Fig. 41), which
is a combina­tion of two individual valves in a single housing. The lower portion or
“body” contains a standard service-brake relay valve, which functions as a relay
station to speed up brake application and release. The upper portion houses a
brake proportioning valve that reduces normal service-brake application pressure
when the tractor is not towing a trailer.
During bobtail operation, this valve reduces stopping distances and gives the
driver greater control over the vehicle.
The driver will note that the brake pedal will have to be pushed farther to apply
sufficient air to stop.
Control
Exhaust
Supply
Delivery
Figure 41. Bobtail proportioning relay valve.
37
Simple tractor-trailer system
In Figure 42, the trailer has been coupled to the tractor and the service and supply
lines of the units have been coupled by using glad hands.
The trailer unit has a reservoir installed. This tank provides a volume of air near
the trailer chambers for normal or emergency braking. The tank is equipped with a
draincock.
A relay emergency valve is mounted on the trailer reservoir. This valve can also
be mounted directly on the trailer frame near the brake chambers. The relay
emergency valve serves three main functions in the system:
1. The relay part of the valve relays air from the trailer reservoir to the trailer-brake
chambers during a brake application. This part of the valve operates like the
relay valve previously discussed. It also provides a quick release of the trailer
brakes.
2. The emergency part of the valve directs trailer reservoir pressure to the
trailer brakes causing an emergency application sometimes referred to as
“dynamiting.” This action occurs automatically in the event of a ruptured or
parted supply line between tractor and trailer, or loss of air from the main
reservoir system. The driver may operate the cab-mounted trailer-supply valve
to cause an emergency application of the trailer brakes.
3. The relay emergency valve has a one-way check valve that stops air in the
reservoir from going back to the source of the supply. The driver has opened
the trailer-supply valve to allow main-reservoir air pressure to be directed
through the tractor protection valve to the trailer. Air pressure passes through
the relay emergency valve to the trailer reservoir. Pressure builds up in the
trailer reservoir to the same pressure as the main reser­voirs on the tractor.
This is known as “charging” the trailer system. The trailer-supply valve remains
in the open position when pressure has built up to between 20 and 60 psi
(138 and 413 kPa), depending on the make.
Drivers can check the operation of the relay emergency valve by closing the
supply valve on the tractor or by disconnecting the supply line between the
tractor and trailer with the supply valve in the open position.
38
39
Trailer
supply
valve
Trailer
supply
Compressor
Safety
valve
Foot
valve
valve
Anti-compound
line
Relay
valve
Glad
hands
Spring-brake and trailer supply circuit
Relay
valve
Supply line
Service line
Figure 42. Typical tractor and trailer charged with air.
Secondary circuit
Primary
reservoir
Gauge
Secondary
reservoir
Low pressure Gauge
switch
Low pressure
switch
One-way check
valve
Two-way
check
valve
Hand
valve
Modulator
valve
Tractor protection valve
Truck/Tractor system
Double check
stoplight switch
One-way check
Supply
reservoir
Double
check
Primary circuit
Governor
Ratio valve
Parking
brake
Air
Dryer
Supply circuit
Brake
chambers
Parking
brake
Trailer reservoir
Relay
emergency
valve
Trailer system
Trailer
reservoir
Brake application – foot valve
Figure 43 illustrates air flow during a brake application made with the foot valve.
Application air has applied the tractor and trailer brakes together. As previously
explained, the two-way check valve has shifted and application air is being directed
through the tractor protection valve to the service line.
Control pressure moves through the service line to act on the relay emergency
valve. Control pressure causes the relay emergency valve to direct reservoir
air from the trailer tank to the trailer-brake chambers. Trailer-brake application
pressure is the same as control pressure, which is the pressure of application air
by the foot valve. In this system, brake lag is minimized.
Release of the foot valve stops the flow of application air. The relay portions of
the valves return to their original positions, stopping the flow of air pressure.
The exhaust ports of the valves exhaust air pressure from the brake chambers,
releasing the brakes.
In this system, the brakes of both units can be released quickly.
40
41
Trailer
supply
valve
Trailer
supply
Compressor
Safety
valve
Foot
valve
Secondary circuit
Primary
reservoir
Gauge
Anti-compound
line
Relay
valve
Glad
hands
Spring-brake and trailer supply circuit
Relay
valve
Supply line
Service line
Figure 43. Tractor and trailer with foot-valve application.
valve
Low pressure
switch
Low pressure Gauge
switch
One-way check
valve
Secondary
reservoir
Two-way
check
valve
Hand
valve
Modulator
valve
Tractor protection valve
Truck/Tractor system
Double check
stoplight switch
One-way check
Supply
reservoir
Double
check
Primary circuit
Governor
Ratio valve
Parking
brake
Air
dryer
Supply circuit
Brake
chambers
Parking
brake
Trailer reservoir
Relay
emergency
valve
Trailer system
Trailer
reservoir
Brake application – hand valve
The driver can use the hand valve to apply the trailer brakes. Air flow is illustrated
in Figure 44. The tractor-protection valve and relay emergency valve are operated
by application air, as explained in the foot-valve application.
Closing the hand valve releases the brakes by closing off application air. Air
pressure in the chambers and lines will exhaust, also as explained in the previous
foot-valve application.
CAUTION: Trailer brakes must not be used to hold a parked
vehicle that is left unattended. Loss of pressure may result in
loss of brakes! Always set the parking brake.
42
43
Air
dryer
Foot
valve
Gauge
Anti-compound
line
Relay
valve
Glad
hands
Spring-brake and trailer supply circuit
Relay
valve
Supply line
Service line
Figure 44. Tractor and trailer with trailer hand-valve application.
Secondary circuit
Primary
reservoir
Low pressure
switch
Low pressure Gauge
switch
One-way check
valve
Secondary
reservoir
Two-way
check
falve
Hand
valve
Modulator
valve
Tractor protection valve
Truck/Tractor system
Double check
stoplight switch
One-way check
valve
Supply
reservoir
Double
check
Primary circuit
Governor
Ratio valve
Parking
brake
Safety
valve
Trailer
supply
Trailer
supply
valve
Compressor
Supply circuit
Brake
chambers
Parking
brake
Trailer reservoir
Relay
emergency
valve
Trailer system
Trailer
reservoir
Emergency applications
A trailer breakaway (Fig. 45) would result in a separation of the service line
and supply line. Sudden loss of air pressure in the supply line triggers the relay
emergency valve, which causes the trailer reservoir to deliver its air directly to the
trailer brake chambers. This places the trailer brakes into emergency application.
Loss of pressure in the supply line also causes the trailer-supply valve to
automatically shift to the closed position.
The tractor brakes are operable, without air loss, because the tractor protection
system has isolated the tractor.
The trailer brakes will remain applied until either the pressure in the trailer reservoir
is drained off or the supply line is repaired and the system is recharged.
44
45
Compressor
Foot
valve
Gauge
Anti-compound
line
Relay
valve
Glad
hands
Spring-brake and trailer supply circuit
Relay
valve
Supply line
Service line
Figure 45. Tractor and trailer breakaway.
Secondary circuit
Primary
reservoir
Low pressure
switch
Low pressure Gauge
switch
One-way check
valve
Secondary
reservoir
Two-way
check
valve
Hand
valve
Modulator
valve
Tractor protection valve
Truck/Tractor system
Double check
stoplight switch
One-way check
valve
Supply
reservoir
Double
check
Primary circuit
Governor
Ratio valve
Safety
valve
Trailer
supply
Trailer
supply
valve
Parking
brake
Air
dryer
Supply circuit
Brake
chambers
Parking
brake
Trailer reservoir
Relay
emergency
valve
Trailer system
Trailer
reservoir
Service-line rupture
If the service line is ruptured or disconnected, no action will take place until a
brake application is made.
In Figure 46, the service line has ruptured and the driver has made a brake
application with the foot valve.
Application air is directed to the control line through the tractor protection valve.
Rupture of the service line will result in the escape of air pressure, if the brake
application is held long enough to cause enough loss of pressure in the tractor
system. This pressure drop causes the tractor protection system to close off,
exhausting the supply line to the trailer. This will cause the trailer brakes to go into
an emergency application.
46
47
Compressor
Hand
valve
Double check
stoplight switch
Secondary circuit
Primary
reservoir
Anti-compound
line
Relay
valve
Glad
hands
Spring-brake and trailer supply circuit
Relay
valve
Supply line
Service line
Figure 46. Tractor and trailer with service-line rupture.
One-way check
valve
Low pressure
switch
Gauge
Low pressure Gauge
switch
One-way check
Secondary
valve
reservoir
Modulator
valve
Tractor protection valve
Truck/Tractor system
Two-way
check
valve
Foot
valve
Supply
reservoir
Double
check
Primary circuit
Governor
Ratio valve
Safety
valve
Trailer
supply
Trailer
supply
valve
Parking
brake
Air
dryer
Supply circuit
Brake
chambers
Parking
brake
Trailer reservoir
Relay
emergency
valve
Trailer system
Trailer
reservoir
Supply-line rupture
Rupture of the supply line (or an uncoupling of the supply line glad hands –
Fig. 47) results in a pressure drop in the supply line between the trailer-supply
valve and relay emergency valve. This triggers the emergency action of the relay
emergency valve, placing the trailer brakes into emergency application. As in the
previous examples, the trailer-supply valve will shift to the closed position.
Operation of the tractor brakes will not be affected if the tractor protection system
is in working condition.
The relay emergency valve must be of the “no-bleed-back” type, so no air is lost
from the trailer.
Note: Depending on the type of tractor protection system used, air loss from the
tractor will stop immediately or it will bleed down to a minimum of 20 psi (138 kPa)
and then shut off. Most newer units will shut off much higher than 20 psi.
48
49
Compressor
Hand
valve
Double check
stoplight switch
Secondary circuit
Primary
reservoir
Low pressure
switch
Gauge
Anti-compound
line
Relay
valve
Glad
hands
Spring-brake and trailer supply circuit
Relay
valve
Supply line
Service line
Figure 47. Tractor and trailer with supply-line rupture.
One-way check
valve
Low pressure Gauge
switch
One-way check
valve
secondary
reservoir
Modulator
valve
Tractor protection valve
Truck/Tractor system
Two-way
check
valve
Foot
valve
Supply
reservoir
Double
check
Primary circuit
Governor
Ratio valve
Safety
valve
Trailer
supply
Trailer
supply
valve
Parking
brake
Air
dryer
Supply circuit
Brake
chambers
Parking
brake
Trailer reservoir
Relay
emergency
valve
Trailer system
Trailer
reservoir
Loss of supply reservoir air
Rupture of the compressor discharge line results in loss of pressure from the
supply reservoir. In Figure 48, the one-way check valves have prevented primary
and secondary reservoir air from escaping back to the supply reservoir and the
ruptured line.
There is sufficient reserve air pressure in the primary and secondary reservoirs
for a limited number of brake applications to stop the vehicle before the parking
brakes are activated.
50
51
Compressor
One-way
check valve
Gauge
Anti-compound
line
Relay
valve
Glad
hands
Spring-brake and trailer supply circuit
Relay
valve
Supply line
Service line
Figure 48. Tractor and trailer with loss of supply-reservoir pressure.
Secondary circuit
Primary
reservoir
Low pressure
switch
Low pressure Gauge
switch
One-way check
valve
Secondary
reservoir
Two-way
check
valve
Modulator
valve
Tractor protection valve
Truck/Tractor system
Double check
stoplight switch
Hand valve
Foot
valve
Supply
reservoir
Double
check
Primary circuit
Governor
Ratio valve
Safety
valve
Trailer
supply
Trailer
supply
valve
Parking
brake
Air
dryer
Supply circuit
Brake
chambers
Parking
brake
Trailer reservoir
Relay
emergency
valve
Trailer system
Trailer
reservoir
Spring-brake trailer system
Components of the spring-brake trailer air system (Fig. 49) are:
• front-service reservoir
• rear-service reservoir
• trailer spring-brake valve
• relay valve (same as on tractor – not an emergency relay valve as used on trailers)
• spring-brake chambers
Service
line
Rear
service
reservoir
Trailer
spring-brake
valve
Supply
line
Relay
valve
Front
service
reservoir
Spring
brakes
Figure 49. Trailer equipped with spring brakes.
The new component – the trailer spring-brake valve (Fig. 50) – is responsible for
several important functions:
• It controls application and release of the trailer’s spring brakes.
• It protects and isolates the front-service reservoir from the rear-service reservoir.
This is an important feature that prevents an automatic application of the springbrakes, even though the trailer’s service reservoir is lost.
• It prevents automatic spring-brake application if the trailer’s supply line has a
gradual leak.
• It will automatically apply the spring brakes if supply pressure is rapidly lost (after
a breakaway).
• Drivers can check the operation of the trailer spring-brake valve by closing the
supply valve on the tractor or by disconnecting the supply line between the
tractor and trailer with the supply valve in the open position.
52
Figure 50. Trailer spring-brake valve.
Section summary
1. What is the purpose of a two-way check valve?
2. Why should the glad hands be protected when not in use?
3. How can a driver control the trailer brakes independently?
4. What are two ways of testing the emergency application of the trailer brakes?
5. Should the hand valve of a tractor trailer unit be used for parking? Why?
6. What is the main purpose of the tractor protection valve?
7. What is the main purpose of the trailer supply valve?
8. Name three functions of the relay emergency valve.
9. Describe the function of the supply line.
10. Describe the function of the service line.
11. What will occur if the supply line ruptures?
12. What will occur if the service line ruptures?
13. What will occur if a brake application is made with a ruptured service line?
14. If the foot valve and the hand valve are operated at the same time, can the
application pressure be greater to the trailer brakes than the truck brakes?
53
6. Checking and adjusting cam-type brakes
(With type-24 and type-30 chambers)
Within an inch (25 mm) of your life
The most common cause of loss of braking is poor brake adjustment. The popular
type-30 air chamber has 2 1/2 in. (63.5 mm) of available stroke. A correctly-adjusted
brake will have 1/2 in. (12.7 mm) to 3/4 in. (19 mm) of slack, leaving two in. (50.8
mm) of reserve chamber stroke. When slack reaches 3/4 in. (19 mm) the brakes
MUST be adjusted. This is the most important 3/4 in. (19 mm) of your life.
Here’s why:
• At an 80 psi (552 kPa) application, a brake chamber with 3/4 in. (19 mm) of
slack will stroke 1 3/4 in. (44.5 mm) due to component stretch. This reduces
reserve chamber stroke to 3/4 in. (19 mm).
• Cast iron expands when heated. On a hot brake drum this can cause the chamber
to stroke a further 1/2 in. (12.7 mm), reducing reserve stroke to 1/4 in. (6.4 mm).
• At high temperature, brake lining wears rapidly. Lining wear the thickness of
three sheets of paper causes the chamber to stroke a further 1/4 in. (6.4 mm),
resulting in the chamber “bottoming out” and a probable runaway.
• Even with cold drums, a vehicle with poorly adjusted brakes will have up to a 75%
longer stopping distance than normal (Table 1).
CAUTION: Under normal light braking conditions even grossly maladjusted
brakes seem to respond satis­factorily. It is only under moderate to heavy
braking that this dangerous condition will become apparent.
Vehicle: 6 x 4 truck | Weight: 55,000 lb. | Speed: 60 mph
Average stopping distance (feet)
Brake lining temperature
150°F
200°F
300°F
400°F
Fully-adjusted brakes:
342 ft
351 ft
366 ft
393 ft
Backed-off to limit:
458 ft
519 ft
625 ft
692 ft
Increase:
34%
48%
71%
76%
Table 1. Lining temperature and stopping distance.
54
Checking
Pull the chamber push rod out to its limit by pulling on the slack adjuster arm or by
prying with a short bar. If push rod travel is 3/4 in. (19 mm) or more, brakes MUST
be adjusted.
Brake adjustment
Slack adjusters
Slack adjusters are mechanical links between the brake-chamber push rod and
the camshaft on cam type brakes. Slack adjusters are not used with wedge-type
brakes.
Slack adjusters are used to manually (Fig. 51) or automatically (Fig. 53) maintain
proper brake chamber stroke and lining-to-drum clearance during normal operation.
Slack adjusters are available in a variety of arm configurations, lengths, torque ratings
and spline types.
The entire slack adjuster operates as a unit, rotating with the brake camshaft as
brakes are applied or released. The most efficient braking occurs when push rod
travel is held to a minimum, therefore it is important that brake adjustments are
made often.
Slack adjuster
Adjusting
nut
Brake cam
Cam roller
Figure 51. Manual slack adjuster.
55
Automatic or self-adjusting slack adjusters
Automatic slack adjusters are designed to continuously and automatically
maintain the brakes in proper adjustment during normal use. However, they must
be checked daily to ensure they are maintaining proper push rod travel – one in.
(25.4 mm) when manually pulled and two in. (50.8 mm) when the brake is applied.
Normally two to four brake applications of 100 psi (689 kPa) per day will keep
the brakes properly adjusted. If they are badly out of adjustment it may take up to
12 brake applications of 100 psi to adjust them. If they are still out of adjustment
a qualified person should repair them. Do not try to adjust them yourself unless
you have been trained by a mechanic or trainer who is familiar with setting up and
backing off this type of automatic slack adjuster.
Automatic slack adjusters must be checked daily.
Manual slack adjuster check – preferred method
With service brakes in the released position, mark the push rod even with the
brake chamber. Make a full brake appli­ca­tion and mark the push rod again.
Measure between the two marks to determine the length of push-rod travel (stroke).
Compare the actual stroke to the recommended maximum stroke of 1 1/2 in.
(38 mm) to determine if brake adjustment is necessary.
Brake adjustment – preferred method
Raise the wheel to be adjusted off the ground so it rotates freely. Turn the slack
adjustment mechanism until the wheel stops. Back off the adjustment until the
wheel turns freely. This would be about one-quarter to one-half of a turn.
This method will result in the shortest possible stroke without the brakes dragging.
Check push rod travel after adjustment.
Brake adjustment – alternate method
Regardless of chamber size or slack adjuster arm length, adjust the slack
mechanism so there is 3/4 in. (19 mm) or less push rod travel when manually (by
hand) extended to place the shoes in contact with the drum.
After adjustment, check for brake contact by gently striking the brake drum with a
metal hammer. When the brake shoes are away from the drum, a ringing sound will
be heard. A dull sound indicates brake drag and that re-adjustment is required until
drag is eliminated.
Check push rod travel after adjustment.
56
Note: If the brakes can’t be adjusted by either of these two methods, inspect the
foundation assembly for worn or broken components.
Service tests
• Apply the brakes and check that the slack adjusters rotate freely without binding.
• Release the brakes and check that the slack adjusters return to their released
position without binding.
• With brakes released, check that the angle formed by the slack adjuster arm
and push rod is greater than 90 degrees (Fig. 52). All slack adjusters should be
adjusted to this same angle.
Note: The practical test will include proper adjustment of a manual slack adjuster
and a verbal explanation of the proper procedure for adjusting an automatic slack
adjuster.
• With brakes applied (20 psi [138 kPa]), check that the new angle is no less than
90 degrees and that all slack adjusters have the same amount of travel.
Released
Applied
Figure 52. Slack adjuster and push rod angle.
Adjusting
linkage
Adjusting crank
Yoke
Worm
Anti-reverse
spring
Clutch spring
Worm shaft
Spline tooth
Drive sleeve
Gear
Figure 53. Automatic slack adjuster.
57
Stroke vs. force
The amount of force available at the push rod is consistent out to two in. (50.8 mm)
of stroke. After two in. (50.8 mm), push rod force drops very quickly (Fig. 54).
Recommended max.
stroke @ 100 PSI
before readjustment
Push rod force (in lb.) at 100
4000
3000
2000
1000
Air chamber
bottom-out
0
0"
1/ "
2
1"
11/2"
2"
21/2"
3"
Push rod stroke
Figure 54. Push rod stroke and force.
Don’t be fooled – check the slack
It is up to YOU, the professional driver, to ensure that your vehicle
has safe, properly adjusted brakes.
Steep downgrade
In some provinces, signs are posted in advance of steep or long downgrades:
These signs indicate that the driver must stop the vehicle in the pull-out area and
inspect the vehicle’s braking system before proceeding. Check:
1. Compressor is maintaining full reservoir pressure.
2. Push rod travel is within limitations on all chambers.
3. No audible air leaks.
4. Glad hands and lines are secure.
5. Drums, bearings and tires are at normal operating temperature.
58
The driver must be aware of the condition of the vehicle’s braking system at all
times. The driver should be able to notice any defects developing in the braking
system and be aware that service or adjustments are required.
The extent of the driver’s responsibility to make repairs will depend on factors such
as the maintenance policy of the company and the driver’s mechanical experience.
TRUCKS
STOP HERE
CHECK BRAKES
STEEP HILL AHEAD
Pre-trip procedure for air single unit
Park the vehicle on level ground with the park brake set, the wheels blocked and
the air tanks drained (if possible):
1. Check security and condition of compressor, belts and air lines under hood.
2. Start engine and let air pressure build up.
3. With wheels blocked, release park brakes.
4. Check brake adjustments (push rod travel) manually. Adjust if necessary.
5. Verbally explain the proper procedure for adjusting an automatic slack
adjuster.
6. Governor operation (be sure spring brakes are released):
• cut-out pressure between 115 and 135 psi (793 and 931 kPa)
• cut-in pressure; fan brakes until compressor cuts in, usually about 20-25 psi less than cut-out pressure, but at a minimum of 80 psi (560 kPa)
7. At maximum pressure:
• ensure the park brake is released
• shut off engine
59
8. Make and hold full foot-brake application:
• maximum air loss after initial application is three psi (20 kPa) in one minute
• listen for audible air leaks
9. With ignition key on, fan brakes to lower air pressure:
• low warning system should operate at minimum 55 psi (379 kPa)
• truck park-brake valve should shut off at minimum 20 psi (138 kPa). On
some vehicles the button may never close however always ensure the
spring brakes have been fully applied
10. Start engine and rebuild air system. Time the build-up from 50 to 90 psi
(345 to 620 kPa). It should be less than three minutes at a maximum of 1,200 rpm.
11. Two final tests:
• apply park brakes and gently try to pull ahead; release park brakes
• move slowly ahead and make foot-brake application
Pre-trip procedure for air combination unit
Park the vehicle on level ground with the park brake set, the wheels blocked and
the air tanks drained (if possible):
1. Check security and condition of compressor, belts and airlines under hood.
2. Start engine and let air pressure build up.
3. With wheels blocked, release park brakes.
4. Check brake adjustments (push rod travel) manually. Adjust if necessary.
5. Verbally explain the proper procedure for adjusting an automatic slack
adjuster.
6. Governor operation (be sure spring brakes are released):
• cut-out pressure between 115 and 135 psi (793 and 931 kPa)
• cut-in pressure; fan brakes until compressor cuts in, usually about 20-25 psi
less than cut-out pressure, but at a minimum of 80 psi (560 kPa)
7. Charge trailer system and rebuild pressure. Shut off engine.
8. Break service line (no air loss should occur).
60
9. Break supply line:
• trailer brakes should apply immediately
• there should be no air loss from trailer line
• air from truck should shut off at a minimum pressure of 20 psi (138 kPa)
10. Reconnect lines, charge trailer and rebuild pressure.
11. At maximum pressure:
• release park brake
• shut off engine
12. Make and hold full foot-brake application:
• maximum air loss after initial application is four psi (28 kPa) in one minute
• listen for audible air leaks
13. With ignition key on, fan brakes to lower air pressure:
• low warning system should operate at a minimum of 55 psi (379 kPa)
• trailer-supply valve should shut off air to trailer at a minimum of 20 psi (138 kPa)
• truck park-brake valve should shut off at minimum 20 psi (138 kPa). On
some vehicles the button may never close, however always ensure the
spring brakes have been fully applied
14. Start engine and rebuild air system on truck only. Time the build-up from 50 to
90 psi (345 to 620 kPa). It should be less than three minutes at a maximum of
1,200 rpm.
15. Four final tests:
• with trailer emergency brakes applied and truck park brakes released, try to
gently pull ahead to test emergency application of trailer brakes
• charge trailer, apply park brakes on the truck only and try to gently pull ahead
• release park brakes, move slowly ahead and apply trailer brakes with hand valve, if equipped
• move slowly ahead and make foot-brake application
Note: Repeat hand and foot-valve test on both sides of the unit checking for
response and, in winter, for frozen wheels.
61
Section summary
1. What is the maximum time permitted for the compressor to build from 50 psi
to 90 psi?
2. What is the maximum pressure loss permitted after the foot brake is fully
applied with the engine shut off?
3. How can trailer-brake holding power be tested?
4. Should all drivers be able to adjust, unassisted, S-cam and drum braking
systems equipped with manual slack adjusters?
5. What is the final brake test that should be made before the vehicle is put into
service?
6. How much push rod travel is allowed before a brake adjustment must be
made?
One person alone is fully responsible to ensure that the braking
system is in safe operating condition before the vehicle moves:
THE DRIVER
62
7. Glossary
Air brakes – A braking system where the brakes are applied by air pressure.
Air over hydraulic brakes – A braking system where the brakes are applied
hydraulically but application is made by air pressure.
Air dryer – A filter that removes most liquid and water vapour from the air before it
reaches the reservoirs.
Air pressure gauge – A dash-mounted gauge telling the driver how much air
pressure is in the primary and secondary reservoirs.
Application pressure gauge – A dash-mounted gauge which tells the driver the
amount of air pressure being delivered when brakes are applied.
Blended air – Air taken from the primary and secondary circuits.
Brake fade – Loss of braking efficiency due to overheating the brakes.
Brake lag – The time required for air to flow through the system when brakes are
applied.
Brake pads – Steel plates with lining which squeeze against the rotor (disc).
Brake pots – A term sometimes used to describe the brake chambers.
Brake drums – Large cast-iron drums located behind each wheel on vehicles
equipped with drum brakes. The brake shoes are forced against them when the
brakes are applied creating friction which stops the vehicle. Brake drums also
dissipate heat.
Brake rotor – A steel disc located behind each wheel on vehicles equipped with
disc brakes. The pads squeeze against it when the brakes are applied.
Brake shoes – Curved steel plates with an outer lining which press against the
brake drums when the brakes are applied.
Breakaway – When the trailer becomes disconnected from the tractor.
Caging – The manual release of the spring brake chamber using the wind-off bolt.
Compounding – Making a service brake application when the parking brake is
applied.
Compressor – A pump operated from the engine which builds and maintains air
pressure.
Cut-in – The point at which the governor allows the compressor to resume pumping.
Cut-out – The point at which the pressure in the system has reached maximum
capacity and the governor stops the compressor from pumping.
63
Diaphragm – A heavy rubber partition in the brake chamber which activates the
brakes when air is forced against it.
Draincocks – Drain valves mounted on each reservoir so they can be drained of
moisture and other contaminants.
Dynamiting – A slang term for an emergency application of the brakes due to
sudden air loss.
Emergency line – A term sometimes used to describe the supply line.
Firewall - The metal panel between the cab and the engine compartment.
Foot valve – A foot operated valve used to apply the brakes.
Front axle ratio valve – A valve which reduces application pressure to the front
brakes.
Fulcrum – The point or support on which a lever pivots.
Glad hands – The coupling devices used to connect the air lines from the tractor to
the trailer.
Hand valve – A hand-operated valve which applies only the trailer brakes.
Loading – A term used to describe cut-in.
Low warning device – A light, buzzer or wig-wag which warns the driver of low air
pressure.
Low warning switch – A switch which activates the low warning device.
Parking brake valve – A yellow diamond-shaped valve mounted on the dash which
activates the parking brakes.
Piggyback – A term sometimes used to describe the spring brake chamber.
Pop-off valve – A term sometimes used to describe the safety valve.
Primary reservoir – A tank that normally supplies air to the brakes on the rear axle(s).
Quick release valve – A valve which exhausts air quickly to reduce release time.
Relay emergency valve – A valve located on or near the trailer reservoir on a trailer
without spring brakes.
Relay valve – A valve which reduces brake lag.
Reservoirs – Tanks where the air is stored.
Safety valve – A valve mounted on the reservoir to prevent over-pressurization.
S-cam – A rotating S-shaped cam which activates the brake shoes.
64
Secondary reservoir – A tank that normally supplies air to the brakes on the
steering axle.
Service brake chamber – A chamber which converts air pressure to mechanical
force when the brakes are applied.
Service line – The rubber hose connecting the tractor and trailer. Air flows through
it when the brakes are applied.
Slack adjusters – Devices used to manually or automatically keep the brakes in
proper adjustment.
Spike – A term sometimes used to describe the hand valve.
Spring brake chamber – A chamber housing a large powerful spring which applies
the brakes if there is an air loss or as a parking brake.
Supply line – The rubber hose connecting the tractor and trailer. Air flows through
it when the trailer supply valve is opened. Sometimes called the emergency line.
Supply reservoir – The first tank where air is stored when it comes from the
compressor. It supplies clean, dry air to the rest of the system.
Tractor protection system – A series of two valves which prevent air loss from the
tractor if a trailer breaks away.
Trailer-supply valve – A red octagonal valve mounted on the dash which supplies
air to the trailer.
Two-way check valve – A valve which ensures a steady supply of air in the event of
an air loss in one system.
µm – A unit of measurement. 2.5 µms = 2.5/1000 of a millimetre or the thickness of
three sheets of paper.
Unloading – A term used to describe cut-out.
Wet tank – A term sometimes used to describe the supply reservoir.
Wig-wag – A warning sign which drops down from above the windshield to notify
the driver the air pressure is too low.
Wind-off bolt – A device on the brake chamber used to manually release the spring
brakes.
65
8. Index
A
Air dryer 20, 21, 63
B
brake drums 3, 4, 7, 63
brake lag 7, 8, 18, 21, 40, 64
Brake pads 63
brake pots 11
brake shoes 8, 12, 56, 63, 64
breakaway 34, 35, 44, 45, 52
C
compounding 28, 32
compressor 9, 10, 14, 15, 16, 17, 20, 21, 24, 50, 59, 60, 62, 63, 65, 68
cut-in 17, 59, 60, 64
cut-out 10, 17, 59, 60, 65
D
diaphragm 6, 8, 11, 14, 27, 29
draincock 10, 38, 64
dynamiting 38
Dynamiting 34, 38, 64
E
emergency line 35, 65
F
foot valve 11, 14, 18, 24, 25, 31, 35, 36, 40, 46, 53
fulcrum 5
G
glad hands 36, 38, 48, 53
H
hand valve 35, 36, 42, 53, 65
L
Loading 64
low warning device 64
66
low warning switch 15, 18
P
park-brake valve 60, 61
Parking-brake system.
piggyback 27
Pop-off 64
primary 22, 24, 25, 28, 30, 31, 32, 33, 35, 50, 63
Q
quick release 38
R
relay emergency valve 38, 40, 42, 44, 48, 53
relay valve 15, 18, 21, 25, 28, 37, 38, 52, 68
Reservoirs 9, 10, 64
S
Safety valve 16, 64
S-cam 12, 13, 62, 64
secondary reservoirs 50, 63
service brake 12, 27, 28, 29, 63
service line 35, 36, 40, 44, 46, 53, 60
slack adjuster 6, 12, 14, 29, 55, 56, 57, 59, 60, 69
Spike 65
spring-brake chamber 27
supply line 33, 34, 35, 36, 38, 44, 46, 48, 52, 53, 61, 64
supply reservoir 15, 16, 18, 24, 50, 65
T
Tractor protection valve 34, 35
Trailer-supply valve 33, 65
Two-way check valve 30, 31, 36, 65
U
Unloading 65
W
Wet tank 65
wig-wag 18, 64
wind-off 27, 31, 63
67
9. Air brake manual summary
1. Name the five basic components of an air brake system.
(1) ____________________________________________
(2) ____________________________________________
(3) ____________________________________________
(4) ____________________________________________
(5) ____________________________________________
2. The maximum air pressure available for a full brake application depends on?
___________________________________________________
3. The most common cause of loss of braking effort on air brake-equipped vehicles is?
___________________________________________________
___________________________________________________
4. How often should the reservoirs be drained of moisture and sludge
accumulation?
___________________________________________________
5. What must an operator do when a low-pressure warning buzzer sounds?
___________________________________________________
6. What is the minimum pressure at which the compressor should cut in? ________ psi
7. If the safety valve on the reservoir blows, it would indicate what?
___________________________________________________
___________________________________________________
8. What is the purpose of a relay valve?
___________________________________________________
___________________________________________________
68
9. How are spring brakes held in the released position?
___________________________________________________
___________________________________________________
10. Spring brakes are most effective as a:
___________________________________________________
___________________________________________________
11. The function of the slack adjuster is:
___________________________________________________
12. Why should spring brakes be released before making a brake application?
___________________________________________________
___________________________________________________
13. Maximum reservoir pressure loss through leaks is:
________ psi in ________ minutes for combination units
________ psi in ________ minutes for single units
14. What factor determines how much heat can be absorbed by the brake drum?
___________________________________________________
___________________________________________________
15. What is the final check to be made by the operator before leaving the yard?
___________________________________________________
___________________________________________________
___________________________________________________
69
10. Conversion charts
kiloPascals (kPa) to pounds per square inch (psi)
kPa
0
1
2
3
4
5
6
7
8
9
psi
psi
psi
psi
psi
psi
psi
psi
psi
psi
—
—
0.15
0.29
0.44
0.58
0.73
0.87
1.02
1.16
1.30
10
1.45
1.59
1.74
1.89
2.03
2.16
2.32
2.47
2.61
2.76
kPa
—
10
20
2.90
3.05
3.19
3.34
3.48
3.63
3.77
3.92
4.06
4.21
20
30
4.35
4.50
4.64
4.78
4.93
5.08
5.22
5.37
5.51
5.66
30
40
5.80
5.95
6.09
6.24
6.38
6.53
6.67
6.82
6.96
7.11
40
50
7.25
7.40
7.54
7.69
7.83
7.98
8.12
8.27
8.41
8.56
50
60
8.70
8.85
8.99
9.14
9.18
9.43
9.57
9.72
9.86
10.01
60
70
10.15
10.30
10.44
10.59
10.73
10.88
11.02
11.17
11.31
11.46
70
80
11.60
11.75
11.89
12.04
12.18
12.33
12.47
12.62
12.76
12.91
80
90
13.05
13.20
13.34
13.49
13.63
13.78
13.92
14.07
14.21
14.36
90
100
14.50
14.65
14.79
14.94
15.08
15.23
15.37
15.52
15.66
15.81
100
Pounds per square inch (psi) to kiloPascals (kPa)
psi
0
1
2
3
4
5
6
7
8
9
kPa
kPa
kPa
kPa
kPa
kPa
kPa
kPa
kPa
kPa
psi
—
—
6.89
13.79
20.68
27.58
34.47
41.37
48.27
55.16
62.05
—
10
68.95
75.84
82.74
89.63
98.53
103.42
110.32
117.21
124.11
131.00
10
20
137.90
144.79
151.68
158.58
165.47
172.37
179.26
186.16
193.05
199.95
20
30
206.84
213.74
220.63
227.53
234.42
241.32
248.21
255.11
262.00
268.90
30
40
275.79
282.69
289.58
296.47
303.37
310.26
317.16
324.05
330.95
337.84
40
50
344.74
351.63
358.53
365.42
372.32
379.21
386.11
393.00
399.90
406.79
50
60
412.69
420.58
427.47
434.37
441.26
448.16
455.05
461.95
468.84
475.74
60
70
482.63
489.53
496.42
503.32
510.21
517.11
524.00
530.90
537.79
544.69
70
80
551.58
558.48
565.37
572.26
579.16
586.05
592.95
599.84
606.74
613.63
80
90
620.53
627.42
634.32
641.21
648.11
655.00
661.90
668.80
675.69
682.58
90
100
689.48
696.37
703.27
710.16
717.05
723.95
730.84
737.74
744.63
751.53
100
70
71
slb142 08/2013 17m 0380
72
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