Many commercial drivers must be qualified
and / or authorized to operate air brake
equipped vehicles and must demonstrate knowledge of air brake systems to ensure
they are fully aware of the proper method of
inspecting an air brake system and identifying
problems that can occur when the system
All Class A commercial drivers are required to pass the Air Brakes Test
and some Class B commercial drivers are required to pass the exam.
Check the Air Brake System
The Seven-Step Air Brake check -DO THIS AT PRETRIP INSPECTION when performing the Skills Test portion of the CDL exam.
Designed to test the governor cut-in and cut-out pressures, air pressure leakage, warning buzzer, brake valves and air pressure rebuild rates. Brakes off means the yellow and/or red valve is pushed in (on = valve out).
1. Engine on / Brakes on – check gauges to make sure governor cuts compressor off at 120 psi. Pump brakes until pressure drops below 100 psi to make sure governor cuts compressor on.
2. Engine off / Brakes off – do not touch brakes – watch gauges to make sure pressure does not drop more than 3 psi in one minute.
3. Press and hold brake pedal – watch gauges to make sure pressure does not drop more than 4 psi in one minute.
4. Turn key on / Engine off / Brakes off – pump brake until pressure drops to 60 psi – warning buzzer should sound at or before 60 psi.
5. Continue pumping brakes until reaching 20 to 40 psi – Emergency (red) and Service (yellow) brake valves should pop out turning brakes on.
6. Rebuild air pressure in tank – hold accelerator at 1500 RPM. Watch air gauges and notice that pressure rate of build between 85 psi and 100 psi should not take more than 45 seconds.
7. Perform tug tests. Tug against Parking Brake only. Tug against Emergency Brake only. Tug against Hand Brake only.
Test service brake by accelerating to 5mph and pressing brake pedal. If vehicle pulls to one side, this indicates a brake alignment problem.
If there is a problem discovered while testing the Air Brakes, do not drive the vehicle. It is your responsibility to make sure it is repaired by a mechanic. Never operate a vehicle that is unsafe to drive.
Another 8 Step Method is also recommended for testing air brakes during a Skills Test for the CDL:
#Step 1- Examine all equipment when testing air brakes, including drums and discs, hoses and chambers, slack adjusters and actuators, relay valves, tanks, pipes and fittings.
- Check that all the brake components, including the air compressor drive belt, are secure. They shouldn't be weakened, worn or damaged.
- Verify that there is a compressor air filter and operational visible brake failure indicators when you test air brakes.
Look for slack adjusters that travel too far when brakes are applied.
- Inspect the compressor hoses and fittings, drive belt and pulley for excessive wear, cracks and tightness.
- Test the rate of air leakage by shutting down the engine with a fully-charged air system. Release the service brake and allow the air system to settle. Time the air pressure drops and record the loss rate. If that rate exceeds the recommended p.s.i. pounds per square inch), look for air leaks and repair them.
- Build up normal air pressure and depress the air supply knob. Turn the engine off. Reduce the air pressure by depressing the brakes a few times.
- Confirm the service brakes are working properly. Attain normal air pressure and discharge the parking brake. Move forward at about 5 mph and firmly depress the brake pedal. Check the brakes if your vehicle pulls to either side, have a strange feel or a slow stopping action.