After connecting the trailer air lines, you should:
Correct. Charge the trailer system, then test the trailer brakes by pulling out the trailer-supply knob to make sure they hold the trailer.
CDL Practice Question #136
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.