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Air Brakes Free CDL Practice Tests
Page 6

Prepare For The Air Brakes Portion Of Your CDL Written Exams

Air Brakes Questions

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After the initial pressure drop, air leakage for single vehicles should be less than:
  • 3 psi in 1 minute
  • 1 psi in 2 minutes
  • 2 psi in 1 minute
  • 4 psi in 1 minute
Click here to look up the answer

From The CDL Manual

Test air leakage rate:

With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

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After the initial pressure drop, air leakage for combination vehicles should be less than:
  • 1 psi in 2 minutes
  • 4 psi in 1 minute
  • 2 psi in 1 minute
  • 3 psi in 1 minute
Click here to look up the answer

From The CDL Manual

Test air leakage rate:

With a fully-charged air system (typically 125 psi), turn off the engine, release the service brake, and time the air pressure drop. The loss rate should be less than 2 psi in 1 minute for single vehicles and less than 3 psi in 1 minute for combination vehicles. Then apply 90 psi or more with the brake pedal. After the initial pressure drop, if the air pressure falls more than 3 psi in 1 minute for single vehicles and more than 4 psi for combination vehicles, the air loss rate is too much. Check for air leaks, and repair before driving the vehicle. Otherwise, you could lose your brakes while driving.

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Generally, air governor cut-in should be at:
  • 40 psi
  • 125 psi
  • 100 psi
  • 120 psi
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From The CDL Manual

Check air compressor governor cut-in and cut-out pressures:

Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer’s specifications.)

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Air governor cut-out should be at around:
  • 125 psi
  • 40 psi
  • 85 psi
  • 100 psi
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From The CDL Manual

Check air compressor governor cut-in and cut-out pressures:

Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer’s specifications.)

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The level at which the air compressor stops pumping air is:
  • Cut-off
  • Pump-out
  • Cut-in
  • Cut-out
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From The CDL Manual

Check air compressor governor cut-in and cut-out pressures:

Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer’s specifications.) Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturer’s specified pressure. The air pressure shown 67 by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut in at about the manufacturer’s specified cut-in pressure.

The pressure should begin to rise. If the air governor does not work as described above, it may need to be repaired. A governor that does not work right may not keep enough air pressure for safe driving.

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To test the parking brake, stop the vehicle, put the parking brake on, and:
  • Gently pull against it in a low gear
  • Gently pull against it in a high gear
  • Have another truck push from behind
  • Pull against it in low gear and high RPM's
Click here to look up the answer

From The CDL Manual

Test parking brake:

Stop the vehicle, put the parking brake on, and gently pull against it in a low gear to test that the parking brake will hold.

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To test the vehicle's service brakes, move the vehicle:
  • In reverse very quickly
  • Forward at 30 mph
  • Slowly backwards, then quickly forwards
  • Slowly forward, at about 5mph
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From The CDL Manual

Test service brakes:

Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph) and apply the brakes firmly using the brake pedal. Note any vehicle “pulling” to one side, unusual feel or delayed stopping action.

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Under normal braking conditions, the vehicle:
  • Should shake a little when you apply the brakes
  • Should be brought to an abrupt stop
  • Should be brought to a smooth, safe stop
  • Should pull to the right a bit
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From The CDL Manual

Normal Stops —

To apply the air brakes during normal stops, push the brake pedal down. Control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, do not push the clutch in until the engine RPM is down close to idle. When stopped, select a starting gear.

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In a controlled braking emergency:
  • Make very small steering wheel movements
  • Apply the brakes as hard as you can without locking the wheels
  • All of these things are part of controlled braking
  • Release and reapply the brakes if the wheels lock or if large steering adjustments need to be made
Click here to look up the answer

From The CDL Manual

Controlled braking:

With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make larger steering adjustments or if the wheels lock, release the brakes. Reapply the brakes as soon as you can.

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Stab braking should only be used:
  • On snow and ice
  • In a panic situation
  • On vehicles without anti-lock systems
  • When traveling downhill
Click here to look up the answer

From The CDL Manual

Stab braking:

Use only on vehicles without anti-lock systems.

  • Apply the brake all the way.
  • Release the brakes when the wheels lock up.
  • As soon as the wheels start rolling, put on the brakes fully again. (It can take up to 1 second for the wheels to start rolling after you release the brakes. If you reapply the brakes before the wheels start rolling, the vehicle will not straighten out.)
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