Arizona CDL Air Brakes: Practice Test

When parking on a level surface, you should use wheel chocks. If the vehicle is not equipped with spring brakes, this is essential to prevent the trailer from moving.

The safety valve protects air tanks and the rest of an air brake system from accumulating too much air pressure. The safety valve is usually set to open at 150 psi.

Some air brake systems contain an alcohol evaporator. This evaporator introduces alcohol into the system to help prevent ice from forming in cold weather.

If a vehicle is equipped, an application pressure gauge shows how much air pressure is being applied to the brakes. This gauge can help a driver identify if their brakes are beginning to fade.

Friction inside a brake drum is caused when the brake shoes and linings push against the inside of the drum. The friction in the drums will slow and stop the vehicle.

A visual low pressure warning signal should activate before air pressure drops to a level between 55 and 75 psi. This signal could be a light, a buzzer, or a wig wag.

Before beginning a trip, it is important to check the brake drums during your walk-around inspection. It is unsafe to drive if any brake drum has a crack more than one half of the width of the friction area.

A vehicle's total stopping distance is made up of perception distance, reaction distance, brake lag distance, and braking distance. With all of these factors included, an air brake-equipped vehicle traveling at a speed of 55 mph under ideal driving conditions will need approximately 450 feet to come to a complete stop.

Air brakes function through the use of compressed air.