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Air brakes practice questions

34 questions from the diesel and heavy truck practice test.

Air brakes practice test

10 questions, graded as you go, with the reason for every answer. No sign-up.

What this covers

Sample questions

The safety (pressure relief) valve on the supply reservoir is normally set to open at about what pressure?

  1. 150 psi
  2. 60 psi
  3. 100 psi
  4. 250 psi
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A. 150 psi

The relief valve protects the system if the governor fails to unload the compressor and is typically set at 150 psi, above the normal cut-out range of about 120 to 135 psi. A setting of 100 psi would be below cut-in and would vent constantly.

Reference: ASE T4 Brakes task list, air supply and service systems (safety valve)

What is the purpose of the one-way check valves at the inlets of the primary and secondary reservoirs?

  1. To limit reservoir pressure to the governor cut-out setting
  2. To drain water from the reservoirs when the compressor unloads
  3. To apply the spring brakes when the reservoir pressure is low
  4. To keep reservoir air from escaping if the supply side fails
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D. To keep reservoir air from escaping if the supply side fails

If the compressor discharge line or supply tank loses pressure, the check valves trap the air already stored in the service reservoirs so the driver still has brakes. They do not regulate pressure.

Reference: ASE T4 Brakes task list, air supply and service systems (reservoirs)

A tractor and semitrailer are tested with the engine off, the system fully charged and the service brakes held fully applied. After the initial drop, what is the generally accepted maximum pressure loss in one minute?

  1. 2 psi
  2. 4 psi
  3. 6 psi
  4. 10 psi
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B. 4 psi

For a combination vehicle the applied leak test limit is 4 psi in one minute (3 psi with the brakes released). The 2 psi figure is the released limit for a single vehicle.

Reference: AAMVA model CDL manual, air brake inspection; ASE T4 Brakes task list

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