Air Brakes · Question #129

Which is part of an air-over-hydraulic brake system?

A. Air booster supplies force to a hydraulic master cylinder
B. Pure hydraulic only
C. Pure air only
D. Vacuum boost

Correct answer: A. Air booster supplies force to a hydraulic master cylinder
Explanation: Air-over-hydraulic systems use air pressure to assist hydraulic brakes.

Why this question matters on the Air Brakes exam

This question is one of the 65 we have for the Air Brakes CDL knowledge test. The official AAMVA Commercial Driver License Manual chapter on Air Brakes covers exactly this topic, and most state DMV exams pull from the same chapter. Memorizing the right answer is fine for passing, but understanding the underlying rule is what keeps you safe on the road and out of trouble with FMCSA inspectors.

If this topic gave you trouble, take the full Air Brakes practice test to drill it across all the related questions. We recommend reading every explanation, even on the questions you got right, because the manual phrasing in the explanation often shows up word-for-word on the real exam.

Take the full Air Brakes practice test →