CDL Practice Question #155
Category: Air Brakes
When the air-pressure protection valve closes during driving (because pressure dropped too low), the trailer:
Correct. When tractor air pressure falls into the 20-45 psi range, the protection valve closes, cutting trailer air, which causes the trailer spring brakes to apply.
Why this question matters
This question is one of 71 in our Air Brakes bank. The reasoning above comes directly from the AAMVA CDL Manual, the source document used by every U.S. state DMV. Memorizing the underlying rule — not just the letter answer — is what separates passing on the first attempt from a return trip and another fee.
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