Modern dual-circuit air-brake systems are required so that:
Correct. Dual systems separate primary and secondary brake circuits so that a failure in one (rear or trailer) still leaves brakes available on the other axle group.
CDL Practice Question #163
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.