The supply pressure gauges show:
Correct. The supply (primary and secondary) pressure gauges show air pressure available for braking — a critical reading before driving and during operation.
CDL Practice Question #105
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.