The service brakes are operated by:
Correct. Service brakes are applied by the foot valve / brake pedal. The yellow and red knobs control parking and trailer-supply functions.
CDL Practice Question #147
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.