Safety first: Why the air brake test is essential after your CDL exam
Drivers are taught to do an air brake test during Entry Level Driver Training (ELDT). You’re often told you must do it in a specific order or fail the Commercial Driver’s License (CDL) test. Many drivers do not know the real reason this test is important. The result is that this crucial test is seldom if ever done. Evidence of this is found when asked to demonstrate an inspection during training initiatives. Many drivers can list the steps, however the order and criteria are often incorrect.
Why we do an air brake test
Drivers skip this test because they do not know the reason the test is important. When surveying drivers about why the test is performed, answers are usually things like:
- So I could pass my CDL test,
- To test my brakes, or
- To be sure I can stop.
While these are valid answers, they are not the root answer. The purpose is to test the condition of the truck’s safety devices. We need to know if the failsafe in the truck operates properly if the air compressor fails. Failure usually happens while the truck is being driven on the highway. Confirming the vehicle warning devices work properly allows you to take action to stop the truck safely.
Preparing for the test
Start the engine and allow the air pressure to rise. As it builds, ensure the pressure builds from 85 to 100 PSI within 45 seconds with no more than 1300 RPMs of the engine. This ensures the compressor is providing air quickly enough to replenish any pressure used. The governor should “cut out,” making an audible pop when air pressure reaches approximately 120 PSI. This will avoid excess pressure on the system.
Leak test
Turn off the engine. When the engine stops, return the key to the on position. Place your foot firmly on the brake pedal and hold for 60 seconds. When the time expires, measure the air loss after the initial drop from brake application. A combination vehicle should not lose more than 4 PSI. A single vehicle cannot lose more than 3 PSI. A successful test ensures the air system will maintain pressure until you become aware of an air compressor failure.
With the wheels chocked and the parking brake released, pump the brake pedal until the air pressure drops to approximately 60 PSI. A warning light and audible alarm should initiate to alert you to the loss of air. The vehicle is now unsafe to operate and you should pull off the road at a safe location as soon as possible.
Button pop out test
Continue to pump the brake pedal, discharging more air from the system. At approximately 20 PSI the parking brake should automatically set. This ensures that, if you are unable to get to a safe place or slow the truck, the parking brake will override your action. This does not guarantee the truck will stop, only that the warning devices are operating properly.
Testing the brakes
Start the engine and return the pressure to 120 PSI. With the parking brakes engaged and chocks removed, tug against the parking brake. This is done by putting the vehicle in gear and attempting to move. The truck should not move. This will confirm the parking brake will hold the truck in place when parked. It also confirms the truck will stop if you fail to take action in an air loss emergency. The final step is to release the parking brake. Drive the truck forward, not exceeding 5 miles per hour, and firmly depress the brake pedal. The truck should stop and not pull left or right.
Key to remember: Ensuring proper operation of safety devices protect drivers, equipment, brand reputation, and the motoring public.