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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 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.

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.

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Service & Parking/Emergency Brakes

Service Brakes

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.

Spring Brakes

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.


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!

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Supply System Components

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 pumping again.

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.

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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.

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Foundation Brakes

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 applied.

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.

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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)

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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!

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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.






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