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Air Brakes Practice Question 24

Restriction L if not passedAir Brakes · Question 24

The required leak-down test before driving consists of:

• ABuilding pressure, then watching it drop with engine off and brakes released for one minute
• BListening for hissing only
• CWatching dashboard lights only
• DDriving and checking pressure

Correct answer: A. Building pressure, then watching it drop with engine off and brakes released for one minute

Why: Build to governor cut-out, shut off the engine, release the brakes, and time the drop for one minute. Then apply the brakes fully and time another minute.

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

• Q20. The alcohol evaporator (used in some systems) helps prevent:
• Q34. Rest of brake fade prevention includes:
• Q47. You should never use the parking brake while:
• Q60. A vehicle that has been sitting unused for several days may have:
• Q11. The supply line (emergency line) on a tractor-trailer is connected by the:

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.