CDL Practice Tests For Air Brakes Page 1

Air brakes use compressed air to make the brakes work. Air brakes are a good and safe way of stopping large and heavy vehicles when the brakes are well maintained and used properly.

Air brake systems combine three braking systems: service, parking and emergency brake systems.

• 1. The service brake system applies and releases the brakes when you use the brake pedal during normal driving.
• 2. The parking brake system applies and releases the parking brakes when you use the parking brake control.
• 3. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in the event of a brake system failure.

Air Compressor Governor—

The governor controls when the air compressor will pump air into the air storage tanks. When air tank pressure rises to the “cut-out” level (around 125 pounds per square inch, or “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.

Air Tank Drains —

Compressed air usually has some water and compressor oil in it, which is bad for the air brake system. For example, the water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure to drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom.

Manually operated by turning a quarter turn or by pulling a cable. You must drain the tanks yourself at the end of each day of driving.

Alcohol Evaporator —

Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working.

Safety Valve —

A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi. If the safety valve releases air, something is wrong. Have the fault repaired by a mechanic.

Brake Pedal—

You apply the brakes by pushing down the brake pedal (also called the “foot valve” or “treadle valve”).

Brake Pedal—

You apply the brakes by pushing down the brake pedal (also called the “foot valve” or “treadle valve”). Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it. If the pressure gets too low, the brakes will not work.

Foundation Brakes —

Foundation brakes are used at each wheel. The most common type is the S-cam drum brake