Modulating valves on the trailer:
Correct. The trailer service brakes are modulated — pressing harder on the foot valve produces more brake-chamber pressure at the trailer.
CDL Practice Question #135
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.