If a trailer breaks away, the emergency brakes on the trailer will be applied because:
Correct. When the air line breaks (or is disconnected), trailer air pressure drops and the trailer spring brakes apply, stopping the trailer.
CDL Practice Question #138
Category: Air Brakes
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.