CDL Air Brakes Test: The Numbers You Need to Memorize
The pressures and leak limits that show up again and again on CDL air brake questions.
Key takeaways
- The governor cuts out at about 125 psi and cuts in around 100 psi.
- The low air warning must come on before pressure falls below 60 psi.
- Spring brakes apply automatically between 20 and 45 psi.
Why these numbers matter
Air brake questions reward exact numbers. If you can recite the pressures and leak limits, you will earn easy points. If you skip them, you will guess.
Compressor and governor
The air compressor fills the tanks, and the governor controls when it stops and starts. The governor typically cuts out, meaning the compressor stops pumping, at about 125 psi, and cuts back in at about 100 psi.
Low air warning
A warning signal, such as a buzzer or light, must come on before the air pressure in the tanks drops below 60 psi. If it comes on while you drive, stop safely as soon as possible.
Spring brakes
The parking brake and emergency brake are spring brakes. They apply automatically when air pressure drops to a range of about 20 to 45 psi. The parking brake knob pops out as they apply.
Leakage limits
In a static leakage test with the brakes released, pressure should not fall more than 2 psi in one minute for a single vehicle or 3 psi for a combination. In an applied leakage test with the pedal pressed, the limits are 3 psi for a single vehicle and 4 psi for a combination.
Other air brake facts
Drain the air tanks to remove moisture and oil. An alcohol evaporator reduces the risk of ice in the lines but does not replace draining. Do not fan the brakes in normal driving, and remember that brake lag is the short delay between pressing the pedal and the brakes applying.
Practice
Write the numbers on a card, drill them for a week and take the air brakes quiz on your state’s CDL practice page. If you skip or fail the air brakes test, your license carries a restriction that bars air-braked vehicles.
Memory tips
Say the numbers out loud in order: 100 in, 125 out, 60 warning, 20 to 45 spring brakes, 2 static, 3 applied. Build a table and fill it in from memory each morning until it is automatic.
Then apply the numbers to short scenarios: what should you do if the low air warning comes on, or if the parking brake pops out? These are the shapes real test questions take.
Beyond the numbers
The test also asks about how air brakes work: the compressor, the governor, tanks, valves and brake chambers. Learn the path air takes from the compressor to the brakes, and it becomes easier to answer questions that describe a symptom and ask for the cause.
Next steps
The bottom line: memorize the six or seven key numbers and the air brake questions become some of the easiest on the test. Take the air brakes quiz for your state on the CDL practice test page.
Frequently asked questions
How many questions are on the air brakes test?
The count is set by the state, and the test is commonly reported as 25 questions. Confirm with your agency.
Do I need it for a Class B?
Yes, if you plan to drive a vehicle with air brakes.
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