The air brake test is a sub-component of the pre-trip inspection that every CDL applicant in an air-brake-equipped vehicle must perform during the skills exam. The test verifies that the vehicle's dual air-brake system is functioning correctly. There are seven discrete steps, and most state DMVs require them in order.
Step 1: Charge the system
Start the engine and let air pressure build to governor cut-out, typically 120 to 140 psi. The compressor will audibly stop loading when cut-out is reached.
Step 2: Test governor cut-in
Apply and release the foot brake repeatedly to drop air pressure. The compressor should resume loading (cut-in) at no more than 100 psi (cut-in is typically 100–125 psi).
Step 3: Test air-loss rate
With the engine off, key on, brakes released: the air pressure should drop no more than 2 psi per minute for a single vehicle, or 3 psi per minute for a combination. With the brake fully applied, the rate is 3 psi/min single, 4 psi/min combination. Excessive air loss indicates a leak that disqualifies the vehicle from operation.
Step 4: Test low-air warning
Continue pumping the brake to drop pressure further. The low-air warning device (buzzer or light) must activate at 60 psi or above.
Step 5: Test spring brake activation
Continue pumping. The spring brakes must apply automatically (the parking-brake knob will pop out) somewhere between 20 and 45 psi.
Step 6: Test parking brake
Restart engine, build pressure, release parking brake, place transmission in low gear, and gently attempt to move forward. The vehicle should not move when the parking brake is then re-applied.
Step 7: Test service brakes
With pressure built, release the parking brake, move forward at about 5 mph, and apply the service brake firmly. The vehicle should stop without pulling to either side and without unusual noises.
Memorize the seven steps in order. Most examiners will mark you down for completing them out of sequence, even if all seven are performed correctly. Drill our Air Brakes study guide for additional concepts.