CDL Air Brakes Test: The Numbers and Parts You Need to Know

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.