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Air Brakes Practice Question 29

Restriction L if not passedAir Brakes · Question 29

The trailer air supply valve (red knob) should be:

• APushed in to apply trailer brakes
• BPulled out to apply trailer parking brakes and supply air to the trailer when pushed in
• CAlways left in
• DPulled out when driving

Correct answer: B. Pulled out to apply trailer parking brakes and supply air to the trailer when pushed in

Why: Push the red knob in to charge the trailer air system once the trailer is coupled. Pull it out to apply trailer parking brakes when stopping or uncoupling.

How this question fits the Air Brakes test

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.

Study technique for this topic

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.

Related Air Brakes practice questions

• Q54. Wedge brake actuators differ from S-cam brakes in that they:
• Q5. Spring brakes typically apply automatically when air pressure falls to approximately:
• Q18. When checking manual slack adjuster travel, the rule of thumb is the chamber pushrod shoul…
• Q32. The proper sequence for the seven-step pre-trip air brake check ends with:
• Q45. The function of the one-way check valve in the air system is to:

Continue your CDL preparation

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.