PracticeDMV

Knowledge area

CDL Air Brakes

20 practice questions cite 9 distinct 49 CFR sections and 2 whole parts. Applicants who fail the air brake part of the knowledge test, or who take the skills test in a vehicle without air brakes, are restricted from driving any CMV with air brakes under 49 CFR 383.95. A skills test in an air brake vehicle includes the air brake checks in 49 CFR 383.113.

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  • Source: PracticeDMV CDL question bank, citations checked against eCFR 2026-09-26
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From this area's questions

What this test covers, by regulation

Knowledge standard cited in this bank: 49 CFR 383.111(b).

SectionTopic in this bankQuestions in our bank
49 CFR 383.111Air brake knowledge, paragraph (b)3
49 CFR 393.50Air supply reservoirs3
49 CFR Part 393Vehicle equipment and connections3
49 CFR 393.43Trailer air supply protection2
49 CFR 393.51Low air pressure warning2
49 CFR Part 396Air tank maintenance2
49 CFR 383.95Air brake restriction1
49 CFR 383.113Vehicle inspection and control skills1
49 CFR 393.41Spring brake operation1
49 CFR 393.48Keeping brakes operational1
49 CFR 396.7Unsafe vehicle operation1

Source: PracticeDMV CDL question bank, citations checked against eCFR 2026-09-26. Subsections of the same numbered section are counted together.

Three questions worked through

These questions explain rules from the air brakes bank.

Why is it dangerous to block or otherwise mechanically hold spring brakes in the released position after an air loss?

  1. It voids the vehicle's registration
  2. It causes the compressor to overheat
  3. It makes the steering wheel harder to turn
  4. It removes the vehicle's last automatic protection, so a driver with no air and no working brakes has no way to stop the vehicle (correct)

Spring brakes applying on air loss is the system's last automatic safeguard against a runaway vehicle. Federal rule requires the brakes a vehicle is equipped with to remain capable of operating, and caging or blocking a spring brake defeats that, which can leave a loaded vehicle on the road with nothing left to stop it.

Source: 49 CFR 393.48.

What should a driver do as soon as the low air pressure warning activates?

  1. Continue the trip but avoid using the brakes
  2. Stop as soon as it can be done safely, since further pressure loss can affect braking (correct)
  3. Turn off the warning and keep driving normally
  4. Shift into a higher gear to build pressure faster

Once the warning is on, the vehicle is already below the safety margin the system is built around, and federal rule bars operating a commercial vehicle in a condition likely to cause a crash or a breakdown. Continuing risks losing normal service brakes or triggering an unplanned spring brake application in traffic.

Source: 49 CFR 396.7.

Under 49 CFR 383.95, which CDL applicant is restricted from operating a commercial motor vehicle with any type of air brakes?

  1. Only one who fails the backing portion of the skills test
  2. Only one who has never driven a truck with air brakes
  3. One who fails the air brake part of the knowledge test, or takes the skills test in a vehicle without air brakes (correct)
  4. Only one who tests in a vehicle with an automatic transmission

Section 383.95(a) sets two triggers: failing the air brake component of the knowledge test, or taking the skills test in a vehicle not equipped with air brakes. Either one puts a restriction on the license that bars driving any CMV with air brakes, including systems that work partly on the air brake principle.

Source: 49 CFR 383.95.

Knowledge area

Take the air brakes test

20questions
16correct to pass

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