You hear a steady "ssss" sound while parked with the engine off. This is most likely:
Correct. A steady hiss with the engine off indicates an air leak — find and repair before driving.
CDL Practice Question #119
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.