Air Brake System Components
12 free questions, part of the Air Brakes CDL practice test
A commercial vehicle's air brake system is really three systems sharing the same parts: the supply system that compresses and stores air, the control system the driver uses to apply the brakes, and the foundation brakes at each wheel that do the actual stopping. ELDT.io's free practice bank has 12 questions on the governor, safety valve, air tank drains, S-cam and wedge brakes, and the tractor protection valve.
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Start Class A CDL Theory ($15)Questions about Air Brake System Components
What does the governor do on an air brake system?
The governor controls when the air compressor builds pressure and when it stops. It cuts the compressor out once tank pressure reaches a set level, typically in the 120 to 135 psi range, and cuts it back in once pressure drops a set amount below that.
Why do you need to drain air tanks on an air brake system?
Air tanks collect moisture and oil off the compressor, and if that builds up it can freeze lines in cold weather or damage valves over time. Tanks with a manual drain valve need to be drained daily, usually at the end of the driving day, unless the vehicle has automatic drain valves installed.
What's the difference between S-cam and wedge brakes?
On S-cam brakes, air pressure pushes a rod that rotates a camshaft, and the S-shaped cam forces the brake shoes out against the drum. Wedge brakes use air pressure to push a wedge directly between the ends of the brake shoes, spreading them apart instead of using a cam.
What is the tractor protection valve for?
The tractor protection valve closes off the tractor's air supply from the trailer if the trailer breaks away or an air line bursts or disconnects. That keeps the tractor's own brakes working even though the trailer loses air and its spring brakes apply automatically.