New! High Road 2.0! We have a new version of this program. You can find it here: High Road 2.0

STEP 7: Final Air Brake Check

Do the following checks instead of the hydraulic brake check described in Section 2: Step 7 – Check Brake System.

Test low pressure warning signal:

Shut the engine off when you have enough air pressure that the low-pressure warning signal is not on. Turn the electrical power on, and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank, or the tank with the lowest air pressure in dual air systems.

If the warning signal does not work, you could lose air pressure and not know it. This could cause sudden emergency braking in a single circuit air system. In dual systems the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

Check that spring brakes come on automatically:

Chock the wheels, release the parking brakes when you have enough air pressure to do it and shut the engine off. Step on and off the brake pedal to reduce the air tank pressure. The “parking brake” knob should pop out when the air pressure falls to the manufacturerʼs specification, usually in a range between 20 and 45 psi. This causes the spring brakes to come on.

Check rate of air pressure buildup:

When the engine is at operating RPM, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems (if the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturerʼs specifications). In single air systems (pre-1975), typical requirements are pressure build-up from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 RPM.

If air pressure does not build up fast enough, the pressure may drop too low during driving, requiring an emergency stop. Do not drive until you get the problem repaired.

Test air leakage rate:

With a fully-charged air system (typically 125 psi), turn off the engine, release the parking 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 manufacturerʼs specifications). Run the engine at a fast idle. The air governor should cut out the air compressor at about the manufacturerʼs 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 manufacturerʼs 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.

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