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2. YOUR BRAKES ARE YOUR BEST FRIEND!! Brake failure is the leading mechanical
cause of truck accidents.
When it comes to vehicle safety, no system is more important than your brakes.
They’re often the only thing between you and disaster.
Having your brakes fail is the most helpless feeling a driver can experience.
Take Care Of Them They Will Take Care of You!

3. Air brake systems are used on most heavy-duty vehicles for
reasons of efficiency and reliability. The major advantage to an air
brake system is that since air never runs out, the air brake system
can always be replenished. An air brake system is also functional
even if it has a small leak.
Heavy trucks use a Dual Air Brake System, which consists of two
separate air brake systems that use a single set of brake controls. It
is designed to retain braking ability in the event one system fails.
Before driving a vehicle with a dual air system, allow time for the air
compressor to build up a minimum of 100 PSI pressure in both the
primary and secondary systems.
The various components in an air brake system work together to
create and maintain a supply of compressed air, direct and control
the flow of that air and to transform air pressure energy into
mechanical force.

4. • The service brake system applies and releases the brakes when
you use the brake pedal during normal driving.
• The parking brake system applies and releases the parking brakes
when you use the parking brake control.

5. • The emergency brake system uses parts of the service and parking
brake systems to stop the vehicle in a brake system failure. When
driving, the emergency brakes are held off by a constant, steady
airflow within a brake chamber, which holds back a very powerful
spring housed inside. When there is insufficient air in the system
to keep the spring in the chamber restrained, the emergency brakes
automatically engage. Spring brakes will apply when air pressure
drops to a range of 20-45psi. (psi = pounds per square inch) When
air pressure falls below 60 psi, a low pressure warning light will
activate and/or an audible buzzer
The braking power of spring brakes depends on the brakes being in
adjustment. If the brakes are not adjusted properly, neither the
regular (service) brakes nor the emergency/parking brakes will work
with full functionality.

6. Supply System Components
Air Compressorpumps air into the air tanks (also referred to as reservoirs), supplying
the compressed air to power the air brake system and other air operated devices.
(Windshield wiper, window) It is lubricated by engine oil or its own oil supply and is
gear driven. A
Safety Valveis usually installed in the reservoir closest to the compressor. Its
function is to protect the system from over pressurization and will release air when
pressure reaches approximately 150 psi.
Air Compressor Governorcontrols when the air compressor will pump air into the
reservoirs. When air tank pressure rises to the set maximum, or the “cut out” level,
(around 125 psi) the governor stops the compressor from pumping air. When the
tank pressure falls to the “cut in” pressure (around 100 psi), the governor allows the
compressor to start pumping again.

7. Air Dryerhelps to keep the system free of contaminants. A filter, typically containing a
desiccant, is installed between the compressor and service reservoir to remove
moisture and oil from the air before it enters the reservoir.
Alcohol Evaporatorinjects alcohol mist into the flow to reduce the risk of freeze-up. It is
not normally used in a vehicle with an air dryer. Its purpose is to prevent moisture from
freezing in the air system in cold temperatures.
Check the alcohol container and fill up as necessary, every day during cold weather.
Daily air tank drainage is still needed to get rid of water and oil. (Unless the system has
automatic drain valves.)
HAND BRAKE(Johnson Bar) Some tractors are equipped with a hand brake, this brake
should never be used to stop or to park. It can be used to test the trailer brakes or to
slide the tandems on trailers equipped to do so.

8. Air Storage Tanksor Reservoirsstore compressed air. The number and size of air tanks
varies. The tanks will hold enough air to allow the brakes to be used several times even
if the compressor stops working. The tractor’s supply air tank receives air from the
compressor and delivers it to the primary and secondary air tanks in the tractor. Most
trailers also have primary and secondary tanks. The reservoir closest to the
compressor is commonly referred to as the “wet tank” because that is where most
moisture condenses. Reservoirs are equipped with
Drain Valves, either manually and/or automatically operated, so that water may be
purged from the reservoirs . Compressed air usually has some water and oil in it,
which tends to collect in the bottom of the air tank. The water can freeze in cold
weather and can lead to brake failure. Drain daily at the end of the day.
Air Supply Gauge(s) in the tractor show how much pressure is in the air tank. All
vehicles equipped with Air Brakes must have a Air Supply Gauge. Some have TWO
gauges some have ONEgauge with TWOneedles.
Application Gauge Shows how much pressure is being applied as you apply the service
brake. Not all tractors are equipped with one.

9. The Control System
One Way Check Valvesin the system prevent air from bleeding back out of a reservoir, or the
compressor.
Quick Release Valvesreduce the chance of brake drag by speeding the process of exhausting air
from brake chambers when the driver lets up on the brake pedal. Relay Valvesspeed brake
application and help to apply equal pressure.
Return Springsretract the brake shoes upon release of the brake pedal.
Two Way Check Valvessense primary and secondary pressure and allow the dominant pressure to
actuate trailer brakes.
Tractor Protection Valveisolates the tractor air system in the event of a trailer breakaway or
dangerous decrease in the tractor’s reserve air. The Treadle Valveis the brake pedal.
Dual Parking Control Valvescan be found on some vehicles, such as busses. They have a separate
air tank, which can be used to release the spring brakes to move a short distance in the case of an
emergency. A
Front Brake Limiting Valvereduces the air to the front brakes by 50% up to approximately 50 PSI,
when it rises proportionately to exert 100% at approximately 60 PSI. Its purpose is to prevent steer
axle brakes from locking up under hard braking.

11. Foundation Brakes
The mechanical components involved in providing braking force are: brake
chambers, brake drums, brake linings, pushrod, s-cam and slack adjusters.
Brake ChamberTransforms air pressure into mechanical force.
Brake DrumsLocated on each end of the vehicle’s axles. Wheels are bolted to the
drums. The braking mechanism is inside the drum. To stop, the brake shoes are
pushed against the inside of the drum.
Brake Linings(or Brake Shoes)

12. Push RodA rod, protruding from a brake chamber, which is connected to the arm of
a slack adjuster via a clevis pin. If the pushrod is in, the brakes are released. If the
pushrod is out, the brakes are applied.
S-CamBrakes One of three types of foundation brakes, 80-85% are S-Cam, the
others being Wedge or Disc.
Slack AdjusterA lever that connects the brake chamber push rod with the foundation
brake camshaft. It provides torque to rotate the brake camshaft when the brake pedal
is depressed. It also provides a means of adjusting clearance between brake shoes
and drum to compensate for lining wear. There are manual and automatic slack
adjusters. Automatics are by far the most common today, but automatic doesn’t
necessarily mean no maintenance. You still need to check for proper adjustment.

14. S-cam Brakes. When you push the brake pedal, air is let into
each brake chamber. Air pressure pushes the rod out, moving
the slack adjuster, thus twisting the brake camshaft. This turns
the s-cam (so called because it is shaped like the letter "S").
The s-cam forces the brake shoes away from one another and
presses them against the inside of the brake drum. When you
release the brake pedal, the s-cam rotates back and a spring
pulls the brake shoes away from the drum, letting the wheels
roll
freely again.

15. Numbers To Know
The system should maintain approximately 100-125 psi at all times. If the system falls
to approximately 60 psi, the low air warning light and/or buzzer should activate. If the
light or buzzer activates, get stopped ASAP! Between 20-45 psi, the spring brakes will
apply and if you are moving, this could be a very rude awakening!
20–45 psi Spring brakes apply
60 psi Low air warning light and/or buzzer
100 psi Compressor “kick-in” (approximate)
125 psi Compressor “kick out” (approximate)
150 psi Safety Valve (protection from over pressurization)

16. A combination vehicle should lose no more than 3 psi with the
engine off and the service brakes released, or 4 psi with the engine
off and service brakes applies.
A straight truck should lose no more than 2 psi with the engine off
and service brakes released, or 3 psi with the engine off and service
brakes applied.
To check the free play in an air brake equipped vehicle, park on level
ground with the wheels chocked and the engine off.

17. SPRING BRAKES
All trucks, truck tractors, and buses must be equipped with emergency
brakes and parking brakes. They must be held on by mechanical force
(because air pressure can eventually leak away). Spring brakes are
usually used to meet these needs. When driving, powerful springs are
held back by air pressure. If the air pressure is removed, the springs put
on the brakes. A parking brake control in the cab allows the driver to let
the air out of the spring brakes. This lets the springs put the brakes on. A
leak in the air brake system, which causes all the air to be lost, will also
cause the springs to put on the brakes. Trailers manufactured prior to 1975
may not have spring brakes.

18. PARKING BRAKE CONTROLS
In newer vehicles with air brakes, you put on the parking
brakes using a diamond-shaped, yellow, push-pull control
knob. You pull the knob out to put the parking brakes
(spring brakes) on, and push it in to release them.

19. COMPOUNDING
Never push the brake pedal down when the spring brakes are on.
If you do, the brakes could be damaged by the combined forces of
the springs and the air pressure.Many brake systems are designed
so this will not happen. But not all systems are set up that way,and
those that are may not always work. It is much better to develop the
habit of not pushing the brake pedal down when the spring brakes
are on.

20. What is a dual air brake system?
What are the slack adjusters?
How can you check slack adjusters?
How can you test the low pressure warning
signal?
How can you check that the spring brakes
come on automatically?
What are the maximum leakage rates?
REVIEW

21. What Is Brake Fade?
Brake Fade is what occurs when the brake drum gets too hot and expands away from the
brake shoes to a degree that pushrod travel is insufficient to fully actuate the brakes. It is
said that they “run out of stroke.” Stroke is the term used in regard to the distance
traveled by the brake chamber push rod or slack adjuster arm during brake application.
The shoes and drum cannot make proper contact and the other brakes will have to work
harder for the brakes that are fading out. These brakes, if forced to continue this for long,
will also eventually get too hot and fade. If they get hot enough, the brakes will smoke. If
they get hotter yet, they may catch fire. Brake fade can be the beginning of brake failure!

22. Brake fade is of particular concern when descending a mountain
grade. If you have to apply more and more pressure to the brake
pedal to maintain the same braking force, the brakes are fading. If
you have a brake application gauge, watch it carefully and often
when descending a grade. Also, know where the Runaway Ramps
are located. If you are only ¼ mile from the bottom of a grade when
you suspect brake fade, you will probably be fine riding it out. If you
know there are eight more miles of grade, you may have a different
situation and important decisions to make.

23. ANTILOCK BRAKING SYSTEMS (ABS)
WHAT IS AN ANTILOCK BRAKING SYSTEM ?
A computerized system that keeps your wheels from locking up during hard brake applications. ABS is an
addition to your normal brakes. It does not decrease or increase your normal braking capability. ABS only
activates when wheels are about to lock up. ABS does not necessarily shorten your stopping distance,
BUT it does help you keep the vehicle under control during hard braking.
HOW DO ANTILOCK BRAKING SYSTEMS WORK?
Sensors and computer logic detect potential wheel lock up, by sensing excessive deceleration rates and noting
substantial differences in wheel speeds. An electronic control unit (ECU) then tells the appropriate modulator
valve(s) to decrease brake pressure to avoid wheel lock up. Brake pressure is adjusted to a level providing the
maximum braking without danger of lockup. ABS works far faster than the driver can respond to potential wheel
lock up. And it only adjusts the brake pressure to the wheels that are in danger of locking up.
At all other times the brake system will operate normally.

24. HOW IS ABS GOING TO HELP ME?
First, it is important to understand what can happen to you without ABS:
When you brake hard on slippery surfaces in a vehicle without ABS, your wheels may lock
up. When your steering wheels lock up, you lose steering control. When your other wheels
lock up, you may skid, jackknife or even spin the vehicle.
ABS helps you avoid wheel lockup. The computer senses impending lockup, reduces the
braking pressure to a safe level, and you maintain control.
You may or may not be able to stop faster with ABS, but you should be able to steer around
an obstacle while braking, and avoid skids caused by over braking.
WHAT IF ABS IS ONLY ON THE TRACTOR, OR ONLY ON THE
TRAILER?
Having ABS on only the tractor, only the trailer, or even on only one axle, still gives you
more control over the vehicle during braking.

25. BRAKE NORMALLY
When only the tractor has ABS, you should be able to maintain steering control and there is less
chance of jackknifing.
BUT keep your eye on the trailer and let up on the brakes (if you can safely do so) if it begins to
swing out.
When only the trailer has ABS, the trailer is less likely to swing out, but if you lose steering
control or start a tractor jackknife, let up on the brakes (if you can safely do so) until you gain
control.

26. HOW SHOULD I BRAKE WITH ABS?
When you drive a tractor-trailer combination with ABS, you should brake as you always
have. In other words:
Use only the braking force necessary to stop safely and stay in control.
And brake the same way, regardless of whether you have ABS on the tractor, the trailer,
or both.
As you slow down, monitor your tractor and trailer and back off the brakes (if it is safe to
do so) to stay in control.
There is only one exception to this procedure. If you always drive a straight truck or
combination with working ABS on all axles, in an emergency stop, you can fully apply
the brakes.

27. EMERGENCY STOPS
If somebody suddenly pulls out in front of you, your natural response is to hit the brakes.
This is a good response if there is enough distance to stop, and you use the brakes
correctly.
You should brake in a way that will keep your vehicle in a straight line and allow you to turn
if it becomes necessary. You can use the "controlled braking" method or the "stab braking"
method.
• Stab Braking
• Apply your brakes all the way.
• Release brakes when wheels lock up.
• As soon as the wheels start rolling, apply the brakes fully again. (It
can take up to one second for the wheels to start rolling after you
release the brakes. If you re-apply the brakes before the wheels
start rolling, the vehicle will not straighten out.)

28. Why should you be in the proper gear before starting down a hill?
What factors can cause brakes to fade or fail?
The use of brakes on a long, steep downgrade is only a supplement to the braking effect of
the engine. True or False?
If you are away from your vehicle only a short time, you do not need to use the parking
brake. True or False?
How often should you drain air tanks?
How do you brake when you drive a tractor-trailer combination with ABS?
You still have normal brake functions if your ABS is not working. True or False?

29. STOPPING DISTANCE
Stopping distance is determined by FOURthings:
Perception Distance –How long it take to perceive danger
Reaction Distance –How long it takes to react
Brake Lag –The amount of time it takes for the air to travel from the
Tanks to the brakes (usually approx¾ of a second)
Braking Distance –Mechanical distance needed to stop

30. 23231524
34342354
454530 95
565638 149
68 6845 214
79 79 53 292
90 90 60 381
101 101 68 482
113 113 75 595
0 100 200 300 400 500 600 700 800 900
Distance (feet)
20
30
40
50
60
70
80
90
100
Speed
Total Stopping Distance
PerceptionReactionBrake LagBraking
896
752
621
503
395
299
215
145
85
301
270
240
211
181
150
120
91
61

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0.750.750.50 2.0
0.750.750.50 2.7
0.750.750.50 3.4
0.750.750.50 4.1
0.750.750.50 4.8
0.750.750.50 5.5
0.750.750.50 6.1
0.750.750.50 6.8
0 1 2 3 4 5 6 7 8 9
Time (seconds)
20
30
40
50
60
70
80
90
100
Speed
Total Stopping Time
PerceptionReactionBrake LagBraking
8.8
8.1
7.5
6.8
6.1
5.3
4.7
4.0
3.4
2.00