Home › Practice › Air Brakes › Question 53
The S-cam in a drum brake system rotates to:
Correct answer: B. Push the brake shoes outward against the drum
Why: When the slack adjuster rotates the S-cam, the cam pushes the brake shoes outward against the drum, creating friction that slows the wheel.
An air brake failure on a loaded combination vehicle can cause runaway, jackknife, or fire. The CDL Air Brakes test exists because air systems behave very differently from hydraulic brakes — drivers must understand pressure build, air loss tolerance, and stopping distance under load.
Memorize the seven steps of the air brake pre-trip check, the leak-down rates for single and combination vehicles, the warning signal pressure threshold, and the spring brake automatic application pressure range. Practice the order of operations — never rely on muscle memory in the booth.
The Air Brakes portion of the CDL knowledge exam covers more than this single question. Use this site's full Air Brakes bank to drill every topic, then test yourself with one of our state-specific practice sets to confirm you are ready for the actual DMV exam in your state. Always check your state's CDL handbook for any local addenda before the test day.
If you are still building your overall CDL foundation, start with the General Knowledge bank — every CDL applicant must pass it — and then work outward to the endorsements you plan to add.
