Commercial Driver’s License (CDL) · Knowledge Test Overview
Air Brakes
On this page 6 sections
In 30 seconds
The Air Brakes knowledge test has 25 questions, and you need 20 correct, or 80 percent, to pass. It is listed for Class A and Class B because those vehicles almost always use full air brake systems. Unlike the endorsement tests, it does not add a privilege; instead, passing it removes a restriction. If you do not pass it, your CDL carries an L restriction that bars you from driving a vehicle with full air brakes.
Why this matters
Almost every Class A and Class B truck on the road stops with air brakes, so the licensing system uses a separate 25-question Air Brakes test to decide whether you are allowed to drive one. Skipping or failing it puts a restriction on your license that keeps you out of those trucks.
The college version
In normal terms
- Twenty-five questions, twenty right, same 80 percent bar as the other tests.
- Skip it or fail it, and your license says "no air brakes" until you pass.
- The questions come almost entirely from the Air Brakes chapter, which is a high-priority chapter, so learn its numbers.
- Take it on the same visit as General Knowledge if you can, because you will need both anyway.
Concepts in this outline
- Air Brakes: 25 questions — A short test built from the high-priority Air Brakes chapter, covering system parts, inspection numbers, warning sequence, and braking technique.
- Passing score: 20 (80%) — Twenty correct out of twenty-five pass, so only five misses are allowed and number questions matter.
- Required for: Class A/B — Listed for the classes whose trucks use full air brakes; skipping or failing it puts an L restriction on the license, while Z marks an air-over-hydraulic test vehicle.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Picture a bike that comes with a lock already on its wheel. The Air Brakes test works in reverse, like that lock. Every new CDL starts out locked away from air brakes. Pass this 25-question test, then pass your skills test in a truck with air brakes, and the lock never appears. Fail it, or skip it, and your license prints with an L restriction. A restriction is a note that limits what you may drive. L means no vehicle with full air brakes. A related Z restriction applies if you tested with air-over-hydraulic brakes, which use air to push on liquid. Either letter locks you out of nearly every big truck. The rules call this test optional, but instructors treat it as required.
The passing score is 20 correct out of 25, which is 80 percent. You can miss only five, so a few number questions can decide the whole test.
The questions come from the high-priority Air Brakes chapter. The compressor is a pump that squeezes air into tanks. The governor tells the compressor when to pump. Reservoirs are the air tanks, and the foot valve is the brake pedal. Spring brakes are strong springs that clamp the brakes shut when air is gone. Expect the inspection numbers. Pressure is measured in psi, a way to measure how hard the air is pushing. The governor cuts in around 100 psi and cuts out around 125 psi. The low-air warning must come on before pressure drops below 60 psi. Spring brakes pop out somewhere between 20 and 45 psi. Expect the leak tests. A single vehicle may lose no more than 2 psi per minute with brakes released. With brakes applied, 3 psi. A combination vehicle is allowed 3 and 4. Pressure should build from 85 to 100 psi in 45 seconds. Expect the difference between service, parking, and emergency brakes. Expect stab braking versus controlled braking. The warning sequence and the three brake systems are the most-missed items.
The test is usually on a computer with an instant score, and it is scored on its own. So you can fail Air Brakes, still get a CLP, and come back for it alone. Retake waiting periods and fees are in your state's CDL manual. The lock can come off later, but passing before your CDL prints is simpler.
The table lists Class A and Class B. A Class C driver in a small hazmat van with hydraulic brakes does not need it. A Class A driver pulling a trailer nearly always does. If your truck might have an air gauge on the dash, take this test and take the lock off.
Worked example
Darnell Whitfield is testing for a Class A license to drive regional freight. He schedules General Knowledge and Air Brakes for the same morning, since both are required for the trucks he wants.
He passes General Knowledge first with 43 out of 50, then starts the 25-question Air Brakes test. Three questions ask for numbers: the pressure below which the low-air warning must sound, the range where spring brakes pop out, and the allowed leak rate on a combination vehicle with brakes released. He answers 60 psi, 20 to 45 psi, and 3 psi per minute, and moves on. He finishes with 22 out of 25. His CLP is issued with no L restriction, and when he later passes his skills test in a tractor-trailer with full air brakes, his CDL prints clean.
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