The "brake-system warning device" must come on no later than:
Correct. A federal-rule low-air-pressure warning device must activate at or before 60 psi.
CDL Practice Question #134
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.