Inspecting Air Brakes
Air brakes have more parts than hydraulic brakes, so a thorough inspection is essential before every trip. A proper air brake inspection confirms that the compressor and governor build and control pressure correctly, that the low-pressure warning and spring brakes work, that the system does not leak too much, and that the mechanical parts at each wheel are in safe condition. The CDL air brake test includes a specific set of checks you must perform in order, ending with a test of the service and parking brakes as you begin to move. This chapter walks through those checks, the pressure numbers you must know, and the wheel-by-wheel items that keep your brakes reliable.
Checking Air Build-Up, Governor, and Leak Rates
With the air system fully charged, you first confirm the compressor and governor work by letting the pressure build to cutout (about 125 psi) and watching the compressor cut back in near 100 psi. In a dual system, the pressure should build from about 85 to 100 psi within about 45 seconds at operating rpm. Then you test how well the system holds air. With the engine off, brakes released, air pressure should not drop more than 2 psi in one minute for a single vehicle (3 psi for a combination). With the brakes fully applied, the drop should be no more than 3 psi in one minute for a single vehicle (4 psi for a combination). Larger losses mean a leak that must be fixed.
Testing the Low-Pressure Warning and Spring Brakes
After the leak tests, you check the safety devices that protect you if pressure falls. With the electrical power on, step on and off the brake pedal to reduce tank pressure (called fanning off the air). The low air pressure warning signal, a light and often a buzzer, must come on before pressure drops below 60 psi. Continue reducing pressure and the spring brakes should apply automatically when pressure reaches the manufacturer's set point, usually in the range of about 20 to 45 psi. On a tractor-trailer you may hear or see the parking brake valve pop out. These tests confirm the vehicle will warn you and then stop itself if air is lost.
Slack Adjusters, Pushrod Stroke, and Wheel Components
During the walk-around you inspect the mechanical parts at each wheel. With the parking brakes released and the wheels chocked, pull hard on each manual slack adjuster you can reach; if it moves more than about one inch from where the pushrod attaches to it, the brakes are likely out of adjustment. Out-of-adjustment brakes and excessive pushrod stroke are common out-of-service violations because they reduce stopping power. Check that brake drums or discs are not cracked and have no loose, broken, or missing parts, and that linings are not dangerously thin or soaked with oil or grease. Also inspect air lines and hoses for cracks, cuts, worn spots, and leaks.
Draining Tanks and the Moving Brake Checks
Air tanks collect water and compressor oil, so tanks without automatic drains must be drained manually as part of inspection; a large amount of oil suggests a compressor problem. The inspection ends with brake checks as you begin to move. Test the parking brakes by setting them and gently trying to pull forward in a low gear; they should hold the vehicle. Test the service brakes by moving forward slowly at about 5 mph and applying the brake pedal firmly; the vehicle should stop straight with no pulling, delay, or unusual pedal feel. For a combination, test the trailer service brakes by moving slowly and applying the trailer hand valve to confirm the trailer brakes engage.
Last updated: July 2026