CDL Practice Tests For Air Brakes Page 6

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.

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.

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.)

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.)

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.

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.

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.

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.

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.)