You need the air brakes test to drive any vehicle with air brakes. Without it, your CDL carries an air brake restriction. Most of the questions use a few key numbers and parts, so learning those is the fastest way to pass. Rules and fees change, so treat this guide as a starting point and confirm everything on the official site before you go.
The parts, in the order the air travels
- Air compressor: driven by the engine, it pumps air into the storage tanks.
- Governor: tells the compressor when to start and stop pumping.
- Storage tanks: hold the air. Drain them regularly to remove water and oil.
- Foot valve (brake pedal): the harder you press, the more air goes to the brakes.
- Brake chambers and slack adjusters: turn the air pressure into braking force at the wheels.
Pressure numbers
- The governor normally cuts in near 100 psi and cuts out near 125 psi.
- The safety relief valve opens at about 150 psi.
- The low air warning must come on before pressure falls below 60 psi.
- Spring brakes start to apply by themselves between about 20 and 45 psi.
- Pressure should build from 85 to 100 psi within 45 seconds with the engine at fast idle (dual air systems).
Leak checks
With the engine off and the brakes released, a single vehicle should lose less than 2 psi in a minute, and a combination less than 3 psi. With the service brake applied, the limits are 3 psi for a single vehicle and 4 psi for a combination.
Habits the test rewards
- Press the pedal smoothly and steadily. Do not pump or fan the brakes.
- If the low air warning comes on, stop and park safely as soon as you can.
- Do not use the parking brake when the brakes are very hot. Use wheel chocks.
- Check slack adjusters: more than about 1 inch of push rod movement means the brakes need adjusting.
Brakes that are out of adjustment are the most common reason air brakes fail. That fact shows up on tests.