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Air Brakes 101
This is a simplified overview on the basics of an air brake system. You
can find more detailed information in the CDL manual.
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
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.
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.
This system applies and releases the tractor and trailer (service) brakes
when the driver pushes the brake pedal. Services brakes are activated when
supplied with air. In simplified terms, when the driver pushes the brake
pedal, air is “pushed” through the airlines to a brake chamber. This air
forces a pushrod out, which in turn pushes a slack adjuster, turning the
camshaft, twisting the S-Cam forcing the brake linings to make contact
with the brake drum. This causes friction, which slows the vehicle. An
Application Pressure Gauge shows how much air pressure is being applied. A
valve on the steering column (known as the Trolley Valve, Hand Brake
and/or Johnson Bar) applies the trailer brakes only, entirely
independently of the tractor brakes. One drawback to air brakes is brake
lag, which is the time required for air to flow through the lines and
force the engagement of the linings to the drum. Though the travel time is
less than a second, air brakes do not apply immediately after the driver
pushes the brake pedal, as they do in a car.
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
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
will sound. Years ago trucks were equipped with a “Wig Wag,” which was a
low air warning device that would drop down directly into the driver’s
view. Though it is doubtful you will drive a truck with a Wig Wag, the CDL
written tests still contain questions about it.
In order to park the vehicle a driver applies the spring brakes by pulling
the valve(s) / button on the dash out. (Trailer brakes, red button;
Tractor brakes, yellow button.) This has the effect of extinguishing the
air from the brake chamber, releasing the spring inside. This forces the
pushrod out, which in turn pushes a slack adjuster, turning the camshaft,
twisting the S-Cam and forcing the brake linings against the brake drum.
Brakes are “set,” “applied” or “engaged” when the driver pulls one or both
dash valves out.
The spring within the brake chamber has approximately 2500 lbs of pressure
behind it, so never attempt to take a brake chamber apart!
The Air Compressor pumps 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 Valve is 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.
The Air Compressor Governor controls 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
An Air Dryer helps 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. An Alcohol Evaporator injects 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.
Air Storage Tanks or Reservoirs store 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. 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. The Air Supply Gauge
(s) in the tractor show how much pressure is in the air tank.
The Control System
One Way Check Valves in the system prevent air from bleeding back
out of a reservoir. Quick Release Valves reduce 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 Valves speed brake
application and help to apply equal pressure. Return Springs
retract the brake shoes upon release of the brake pedal. Two Way Check
Valves sense primary and secondary pressure and allow the dominant
pressure to actuate trailer brakes. The Tractor Protection Valve
isolates the tractor air system in the event of a trailer breakaway or
dangerous decrease in the tractor’s reserve air. The Treadle Valve
is the brake pedal. Dual Parking Control Valves can 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 Valve reduces 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.
The mechanical components involved in providing braking force are: brake
chambers, brake drums, brake linings, pushrod, s-cam and slack adjusters.
Brake Chamber Transforms air pressure into mechanical force.
Brake Drums Located 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)
Push Rod A 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
S-Cam Brakes One of three types of foundation brakes, 80-85%
are S-Cam, the others being Wedge or Disc.
Slack Adjuster A 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.
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
85 psi Bus Warning
100 psi Compressor “kick-in” (approximate)
125 psi Compressor “kick out” (approximate)
150 psi Safety Valve (protection from over pressurization)
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!
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.
Hot brakes smell very, very bad. After smelling them once, you will never
forget what the smell is like. Some drivers joke about “smoking their
brakes” - and most every driver has done it at some time, but hopefully
only once and hopefully, it taught him/her their lesson!
This article is an excerpt,
modified for the web, from "Driver's ABC's, Surviving the First Year
Guidebook Fourth Edition," Copyright 2002, All Rights Reserved. This
article may not be reproduced, nor distributed without the explicit
permission of Creative Curriculum FTTI.