Home › CDL Practice Tests › Air Brakes Questions › Question 231
A trailer breaks away from the tractor and tears both air lines loose. What happens on the tractor side in the first moments?
- A.The tractor loses all of its air instantly through the torn lines and cannot brake at all
- B.The trailer supply valve pops out and the tractor protection valve closes, keeping enough air in the tractor's tanks for a controlled stop
- C.The trailer's spring brakes stay released because the lines are open
- D.The tractor's compressor shuts down to protect itself from overspeeding
Correct answer
B. The trailer supply valve pops out and the tractor protection valve closes, keeping enough air in the tractor's tanks for a controlled stop
Why
When trailer supply pressure collapses, the supply knob pops out and the protection valve slams shut, sealing the tractor's reservoirs off from the torn lines. That reserve is what lets the driver still brake the tractor to a stop, while the trailer's own spring brakes apply automatically.
Manual topic: Air Brakes - Tractor Protection Valve · From the Air Brakes practice bank (AB-231).