CDL Air Brakes Practice Test
Updated October 27, 2026
The Air Brakes knowledge test is required for most Class A and Class B drivers, unless you accept an air brake restriction on your license. It usually has 25 questions, and most states require 80% to pass. It’s the test that catches the most people, because it asks for exact numbers. Learn the key numbers below, then try the free questions.
The numbers on this page follow the model CDL manual most states use. Check your state’s manual for any differences.
Key numbers to know
| What | Number |
|---|---|
| Governor cut-out (compressor stops pumping) | About 125 psi |
| Governor cut-in (compressor starts pumping again) | About 100 psi |
| Low air pressure warning comes on | Before pressure drops below 60 psi |
| Spring brakes come on (tractor and straight trucks) | Between 20 and 45 psi |
| Pressure build-up time (dual air system) | From 85 to 100 psi within 45 seconds |
| Leakage rate, brakes released | Under 2 psi per minute (single vehicle), 3 psi (combination) |
| Leakage rate, brakes applied | Under 3 psi per minute (single vehicle), 4 psi (combination) |
The air brake check
On the pre-trip inspection, you test the air brake system in a set order: the low air pressure warning, the spring brakes coming on, the build-up rate, the leakage rate, the governor cut-in and cut-out, and the parking and service brakes.
Free CDL Air Brakes practice questions
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At about what pressure should the air compressor governor stop the compressor from pumping air (cut-out)?Answer: CThe governor usually stops the compressor at about 125 psi (cut-out) and starts it again at about 100 psi (cut-in). The safety valve opens at about 150 psi to protect the system if the governor fails.
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The low air pressure warning signal must come on before the pressure in the service air tanks falls below:Answer: CThe low air pressure warning, usually a red light and a buzzer, must come on before pressure drops below 60 psi. If it comes on while driving, stop safely as soon as possible.
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On a tractor or straight truck, the spring brakes should come on automatically when air pressure falls to:Answer: AWhen air pressure falls to between 20 and 45 psi, the spring brakes come on automatically. Stop before that happens, because the vehicle can stop suddenly in an unsafe place.
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With a dual air system, how long should it take for pressure to build from 85 to 100 psi at fast idle?Answer: AIn a dual air system, pressure should build from 85 to 100 psi within 45 seconds. A slower build-up can mean a leak or a problem with the compressor.
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With the engine off and the brakes released, the air pressure in a single vehicle drops 4 psi in one minute. What does this mean?Answer: BWith brakes released, the leakage rate should be less than 2 psi in one minute for a single vehicle (3 psi for a combination). A 4 psi drop means there's too much leakage, so check for leaks and fix them before driving.
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Why should air tanks be drained?Answer: CWater and oil from the compressor collect in the bottom of the air tanks. Water can freeze in cold weather and cause brake failure, so drain the tanks completely, as your vehicle's procedure requires.
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Which color is usually used for the emergency (supply) air line between a tractor and a trailer?Answer: DThe emergency (supply) line is usually red, and the service (control) line is usually blue. Crossing them up can mean the trailer brakes don't work, so check the connections carefully when coupling.
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With air brakes, what extra distance adds to total stopping distance compared with hydraulic brakes?Answer: CAir brakes take a moment to work after you press the pedal, because air must flow through the lines. The distance traveled during that delay is brake lag distance.
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When checking a manual slack adjuster with the engine off and the brakes released, the push rod should not move more than about:Answer: BIf you can pull the slack adjuster and the push rod moves more than about 1 inch, the brakes probably need adjusting. Many vehicles have automatic slack adjusters, but they still need checking.
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What does the safety valve in an air brake system do?Answer: DThe safety valve protects the tank and system from too much pressure, usually opening at about 150 psi.
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In a dual air brake system, what happens if one of the two systems loses air?Answer: AA dual air system has two separate systems controlled by one brake pedal. If one fails, the other still provides some braking, but you should stop safely and get it repaired.
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Why should you avoid using the parking brakes when the brakes are very hot?Answer: BThe manual warns against using the parking brakes when the brakes are very hot, such as after a steep downgrade, because the heat can damage them. Use wheel chocks if needed.
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What should you do if the low air pressure warning comes on while driving?Answer: CLow air pressure can lead to sudden braking when the spring brakes come on. Stop and park safely while you still have control.
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Brake fade is usually caused by:Answer: DExcessive braking causes heat, and overheated brakes lose effectiveness (fade). Use a low gear on downgrades so the engine helps slow the vehicle.
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What is the purpose of an alcohol evaporator in some air brake systems?Answer: AAn alcohol evaporator adds alcohol to the air system to reduce the risk of ice forming in valves and lines in cold weather.
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The front brake limiting valve on older vehicles is used to:Answer: BOlder vehicles may have a front brake limiting valve with a "slippery" setting to reduce front braking. Tests have shown front wheel braking is good in all conditions, so many newer vehicles automatically adjust instead.
Tips
Make a card with the numbers in the table above and quiz yourself until they’re automatic. Then learn what each part does: the compressor, governor, air tanks, drains, safety valve, brake chambers, slack adjusters and spring brakes. Questions often describe a symptom and ask which part is at fault.
Pass your CDL permit test on the first try
- All 215 questions for General Knowledge, Air Brakes, Combination Vehicles and Hazmat
- An explanation for every answer, from the CDL manual and federal rules
- Unlimited practice tests for each knowledge test, scored like the real one (80% to pass)