The "tractor protection valve" closes (cuts off air to the trailer) automatically when:
Correct. The tractor protection valve protects the tractor from a runaway air loss in case of a trailer break-away or air leak — it closes between 20 and 45 psi.
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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