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Air brakes practice questions

40 questions from the cdl general knowledge, air brakes, combination practice test.

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Air brakes practice test

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

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Sample questions

At about what tank pressure does the governor stop the compressor from pumping air (cut-out)?

  1. 85 psi
  2. 100 psi
  3. 125 psi
  4. 150 psi
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C. 125 psi

The governor cuts the compressor out at around 125 psi. About 100 psi is the cut-in level where pumping starts again, and 150 psi is where the safety valve opens.

Reference: CDL Manual Section 5.1, The Parts of an Air Brake System

At about what tank pressure does the governor let the compressor start pumping again (cut-in)?

  1. 20 psi
  2. 60 psi
  3. 100 psi
  4. 125 psi
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C. 100 psi

When tank pressure falls to about 100 psi the governor cuts the compressor back in. The 125 psi level is the cut-out point where it stops pumping.

Reference: CDL Manual Section 5.1, The Parts of an Air Brake System

Why must air tanks be drained?

  1. Stale air loses its braking pressure
  2. Draining resets the governor cut-out pressure
  3. Full tanks slow the compressor down
  4. Water and compressor oil collect in them
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D. Water and compressor oil collect in them

Compressed air carries some water and compressor oil, which settle in the bottom of the tanks. In cold weather the water can freeze and cause brake failure, so the tanks have to be drained.

Reference: CDL Manual Section 5.1, The Parts of an Air Brake System

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