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Page 1 Air Brakes Test Study Guide for the CDL

General Information

Knowledge of the air brake system is essential for anyone desiring to obtain a Commercial Driver’s License that allows operation of a truck with air brakes. This study guide outlines the information you need to know to safely man such a vehicle.

Parts of an Air Brake System

The air brakes system has three parts: the service, parking, and emergency brake systems. Within these systems, there are many main parts with which one needs to be familiar.

  • The air compressor pumps air into the storage tanks.

  • The air compressor governor manages when the compressor will pump air into the storage tanks.

  • The air storage tanks hold the compressed air used in the braking system.

  • The air tank drains drain built up water or oil in the bottom of the tanks.

  • The alcohol evaporator puts alcohol into the air system to reduce the risk of ice.

  • The safety valve automatically opens at a set PSI (pounds per square inch), usually 150, to prevent too much pressure in the air brakes system.

  • The brake pedal engages the air brakes when pressed.

  • Foundation brakes are the brakes used at each wheel. They are most commonly S-cam brakes but can also be wedge brakes or disc brakes.

  • Supply pressure gauges are connected to the air tanks and tell the pressure of each tank.

  • The application pressure gauge displays how much pressure is being applied to the brakes. The low air pressure warning alerts when the air brake pressure falls below a safe amount (usually 60 PSI).

  • The stop light switch turns on the brake lights when applying the brakes, alerting anyone behind that the vehicle is slowing.

  • The front brake limiting valve is only on vehicles made before 1975. When set to “slippery,” it cuts the standard air pressure to the front brakes in half.

  • Spring brakes are used for the emergency and parking brake systems. Powerful springs are held back by air pressure, and when that air pressure is released, the springs allow for braking.

  • The parking brake controls are usually a yellow, diamond-shaped, push-pull control knob. On older vehicles, the parking brake may be controlled by a lever.

Dual Air Brake Systems

Dual air brake systems are used by most new vehicles. In a dual system, there are two separate air brake systems, each with its own lines, hoses, air tanks, etc., but both run by the same set of brake controls. Typically one system (the primary system) controls the regular brakes on the rear axle/s and the other system (the secondary system) controls the brakes on the front axle (and maybe one rear axle).

Operating the Air Brakes

In a normal stop, to operate air brakes you simply push the brake pedal down using controlled pressure. To perform an emergency stop, there are two methods you can use.

  • The first is controlled braking, which means applying the brakes as hard as possible without locking the wheels.

  • The second, stab braking, means pressing on the brake pedal until the brakes lock up, releasing the brakes, and then as soon as the wheels begin rolling again, pressing the pedal back down as hard as you can.

Other Considerations

There a few special considerations you should know about air brakes: the stopping distance, parking, and low pressure warnings.

  • The stopping distance for a vehicle with air brakes is slightly longer than for a normal vehicle because of the added delay for the air to flow through the lines to the brakes. Because of this, there are circumstances that may require extra care when you are coming to stop. For example, braking on downgrades can cause excessive heat in the brakes, causing them to fade. In this case, it is best to use a lower gear and to drive slowly enough that a light, steady use of brakes will keep you from increasing speed. On- and off-braking alone has been proven ineffective and should not be used as an alternative to downshifting.

  • If you ever get a “low pressure warning” as you drive, pull over and park as soon as it safe to do so. A low pressure warning may indicate a leak in the air brakes system, and you will only be able to do a controlled brake if the pressure in the air tanks stays above a certain level.

  • Finally, in order to safely park your vehicle, the parking brakes must be engaged. The only exceptions are when the brakes are very hot or if the brakes are very wet in freezing conditions. In these cases, it is best to use wheel chocks to hold the vehicle.