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Air Brakes Practice Question 13

Restriction L if not passedAir Brakes · Question 13

The trailer hand valve (trolley valve, Johnson bar) should be used:

• ARoutinely for slowing down
• BOnly to test trailer brakes; not for service braking
• CFor all stops in traffic
• DOnly at parking

Correct answer: B. Only to test trailer brakes; not for service braking

Why: Using the hand valve as a primary brake increases jackknife risk because it brakes only the trailer wheels. Use the foot brake for service braking; the hand valve is mainly for testing.

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

• Q46. A hissing sound that continues after the engine is shut off and brakes released usually in…
• Q59. When you apply the trailer hand valve, you should not use it to:
• Q10. The proper braking technique for a long downgrade is:
• Q24. The required leak-down test before driving consists of:
• Q37. A water-saturated air brake system can lead 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.