When you supply air to the trailer for the first time after coupling, you should:
Correct. Charge the trailer system. Then test the trailer brake function by tugging gently or pulling the trailer-supply knob to confirm trailer brakes apply.
CDL Practice Question #192
Category: Combination Vehicles
Why this question matters
This question is one of 55 in our Combination Vehicles 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.