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5.3.3 – Step 7 Final Air Brake Check (continued)

Test Air Leakage Rate. With a fully charged air system (typically 125 psi), turn off the engine, release the parking brake (push the button in) and time the air pressure drop. The loss rate should be less than 2 psi in one minute for single vehicles and less than 3 psi in one minute for combination vehicles.

With the air pressure built up to governor cut-off (120-140 psi), shut off the engine, chock your wheels (if necessary), release the parking brake (all vehicles) and the tractor parking valve (combination vehicle), and fully apply the foot brake. Hold the foot brake for one minute. Check the air gauge to see if the air pressure drops more than 3 pounds in one minute (single vehicle) or 4 pounds in one minute (combination vehicle).

If the air pressure falls more than 3 psi in one minute for single vehicles (more than 4 psi for combination vehicles), the air loss rate is too much. Check for air leaks and fix before driving the vehicle. Otherwise, you could lose your brakes while driving.

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 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 fixed. A governor that does not work properly may not keep enough air pressure for safe driving.

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.

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. This test may show you problems, which you otherwise would not know about until you needed the brakes on the road.

Multiple-Choice Questions:

Question #280 (1 of 5)

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Which of the following describes the air compressor cut-in and cut-out pressure test?

  • None of these are correct
  • 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 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.
  • With the engine off, hold down the foot brake and make sure the compressor kicks on right away. If it does not, pump the brake a few times to let some air out of the system. If the compressor comes on when the air pressure drops, the governor is not working.
  • Run the engine at a fast idle. Make sure the air compressor does not come on until the air pressure is above 200 psi. Make sure it does not stop pumping until the air pressure is below 50 psi

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 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.

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Question #278 (2 of 5)

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Air Leakage Rate Test: With the air pressure built up to governor cut-off (120-140 psi), shut off the engine, chock your wheels (if necessary), release the parking brake (all vehicles) and the tractor parking valve (combination vehicle), and fully apply the foot brake. Hold the foot brake for one minute. The loss rate should be:

  • less than 2 psi in one minute for single vehicles and less than 3 psi in one minute for combination vehicles.
  • less than 10 psi in one minute for single vehicles and less than 15 psi in one minute for combination vehicles.
  • None of these are correct
  • less than 3 psi in one minute for single vehicles and less than 4 psi in one minute for combination vehicles.

Test Air Leakage Rate. With the air pressure built up to governor cut-off (120-140 psi), shut off the engine, chock your wheels (if necessary), release the parking brake (all vehicles) and the tractor parking valve (combination vehicle), and fully apply the foot brake. Hold the foot brake for one minute. Check the air gauge to see if the air pressure drops more than 3 pounds in one minute (single vehicle) or 4 pounds in one minute (combination vehicle).

There are two tests for air leakage. One test uses the foot brake, the other does not.

Remember these numbers:

1) Testing without using the foot brake, the pressure can only drop 2 psi for single vehicles, 3 psi for combination vehicles.

2) Testing with the foot brake, the pressure can only drop 3 psi for single vehicles, 4 psi for combination vehicles

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Question #281 (3 of 5)

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When testing the service brakes, you 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. What are you checking for?

  • Make sure the ABS light flashes on and off while you're pressing the brake pedal. If the light is not flashing, something is wrong with the service brakes
  • The vehicle should not stop unless you're using the trailer hand valve
  • You should have a delay of 2 - 3 seconds from the time you hit the foot brake until you feel the brakes grab. Any sooner than this and the brakes are out of adjustment
  • Any vehicle “pulling” to one side, unusual feel or delayed stopping action

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. This test may show you problems, which you otherwise would not know about until you needed the brakes on the road.

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Question #277 (4 of 5)

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Air Leakage Rate Test: With a fully charged air system (typically 125 psi), turn off the engine, release the parking brake (push the button in) and time the air pressure drop. The loss rate should be:

  • less than 3 psi in one minute for single vehicles and less than 4 psi in one minute for combination vehicles.
  • less than 10 psi in one minute for single vehicles and less than 15 psi in one minute for combination vehicles.
  • None of these are correct
  • less than 2 psi in one minute for single vehicles and less than 3 psi in one minute for combination vehicles.

Test Air Leakage Rate. With a fully charged air system (typically 125 psi), turn off the engine, release the parking brake (push the button in) and time the air pressure drop. The loss rate should be less than 2 psi in one minute for single vehicles and less than 3 psi in one minute for combination vehicles.

There are two tests for air leakage. One test uses the foot brake, the other does not.

Remember these numbers:

1) Testing without using the foot brake, the pressure can only drop 2 psi for single vehicles, 3 psi for combination vehicles.

2) Testing with the foot brake, the pressure can only drop 3 psi for single vehicles, 4 psi for combination vehicles

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Question #279 (5 of 5)

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When should the air compressor start and stop pumping?

  • It should start at about 150 psi and stop at about 200 psi.
  • It should start at about 50 psi and stop at about 75 psi.
  • None of these are correct
  • It should start at about 100 psi and stop at about 125 psi.

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 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.

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