The spring brakes:
Correct. Spring brakes use heavy springs that apply the brake when air pressure is released or drops below 20-45 psi — they serve as both parking and emergency brake.
CDL Practice Question #109
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.