Free practice test
CDL Air Brakes practice test
The free test below follows the real Air Brakes test: 25 questions, 20 right to pass, 25 to 30 minutes. It is the test new drivers underestimate most, because it has its own words and exact numbers.
FirstTry CDL is not affiliated with any DMV, FMCSA or government agency. Practice questions are exam-style questions built from the official CDL manual, not actual test questions.
How the Air Brakes test is scored
You need 20 of 25 right, which is 80%. That means you can miss 5. Anyone who will drive a vehicle with air brakes. If you take your skills test in a truck with air brakes and have not passed this test, your CDL gets an air brake restriction. Most truck jobs need that restriction off.
What is on the Air Brakes test
- How the system works: compressor, governor, air tanks and brake chambers
- Dual air brake systems, and the spring brakes that act as parking and emergency brakes
- Warning devices, like the low air pressure warning
- Inspecting air brakes: the leakage test and pressure checks
- Using air brakes: normal stops, emergency stops and parking
What trips drivers up
The low air pressure warning
It must come on before the air in the tanks falls below 55 psi in the current model manual. Some older manuals print 60 psi, so check the number in your state's manual.
Spring brakes
In most vehicles the spring brakes come on when pressure falls to somewhere between 20 and 45 psi.
The governor
The governor tells the compressor when to stop and start. It cuts in at about 100 psi and cuts out at around 120 to 140 psi.
Air loss rate
With the engine off and the brakes released, a single vehicle should lose less than 2 psi in one minute. A combination vehicle should lose less than 3 psi.
Brake lag
Air brakes take a moment to act after you press the pedal. That delay is part of your total stopping distance.
Numbers come from the CDL manual and federal rules. A few differ by state or manual edition, so check your own state's manual. See our sources.
More practice tests
Common questions
How many questions are on the CDL Air Brakes test?
The national default is 25 questions, and you need 20 right (80%) to pass. Some states differ, so check your manual.
Is the Air Brakes test hard?
Many drivers say it is the hardest knowledge test because it uses technical words and exact numbers. Learn the parts, then learn the numbers, and take the test with explanations turned on.
What is the air brake restriction?
If you do your skills test in a vehicle with air brakes without passing the Air Brakes knowledge test, your license gets a restriction. You can only drive vehicles without air brakes until you pass.
Do I need the Air Brakes test for Class B?
If the vehicle you will drive has air brakes, yes. Many Class B trucks and buses do. Ask your employer or school which vehicle you will test in.
At what pressure must the low air warning come on?
Before the pressure in the tanks falls below 55 psi in the current model CDL manual. Some older state manuals print 60 psi. Learn the number in your own state's manual.