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CDL Air Brakes Test

Inspecting air brakes: practice questions

Our CDL Air Brakes bank has 37 questions on this topic. Here are 5 of them with answers; the full mock exam draws from every topic, like the real one.

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Question 1

With the foot brake fully applied for one minute (engine off), what pressure drop is too much on a combination vehicle?

  1. More than 3 psi
  2. More than 4 psi
  3. More than 2 psi
  4. More than 6 psi
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B. More than 4 psi — For combination vehicles the applied leakage limit is 4 psi in one minute (CDL Manual 5.3.3)

Question 2

In a dual air system at operating rpm, how fast should pressure build from 85 to 100 psi?

  1. Within 3 minutes
  2. Within 45 seconds
  3. Within 15 seconds
  4. Within 2 minutes
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B. Within 45 seconds — Pressure should build from 85 to 100 psi within 45 seconds in dual air systems (CDL Manual 5.3.3)

Question 3

How do you test the governor cut-in point?

  1. With the engine off, open a drain valve slowly
  2. Pull the parking brake knob out and in
  3. With the engine idling, step on and off the brake until pressure begins to rise
  4. Rev the engine until the safety valve opens
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C. With the engine idling, step on and off the brake until pressure begins to rise — With the engine idling, step on and off the brake to lower tank pressure. The compressor should cut in at about the specified pressure, and pressure should begin to rise (CDL Manual 5.3.3)

Question 4

When doing the applied leakage test on a combination vehicle, what must you release before applying the foot brake?

  1. Only the trailer hand valve
  2. The governor and the safety valve
  3. The parking brake and the tractor protection valve
  4. The front brake limiting valve
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C. The parking brake and the tractor protection valve — With pressure at governor cut-off, shut off the engine, chock the wheels, release the parking brake and the tractor protection valve (combination), then apply the foot brake (CDL Manual 5.3.3)

Question 5

What fully charged pressure does the manual give as typical for starting the static leakage test?

  1. About 55 psi
  2. About 125 psi
  3. About 85 psi
  4. About 150 psi
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B. About 125 psi — Test air leakage with a fully charged system, typically 125 psi (CDL Manual 5.3.3)

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