CDL Air Brakes Practice Test
Fifteen original practice questions on how air brake systems work, how to check them and how to brake safely. Each answer explains the reason.
Pick an answer for each question. You’ll see right away if it’s correct and why.
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Answer: B. About 125 psi The governor cuts the compressor out at around 125 psi on a typical system. Always check the manufacturer's specification for your truck.
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Answer: A. About 100 psi When tank pressure falls to around 100 psi, the governor lets the compressor pump again. That keeps the tanks between roughly 100 and 125 psi.
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Answer: C. 150 psi The safety valve protects the system from too much pressure and is usually set at 150 psi. If it releases air, something is wrong and a mechanic should fix it.
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Answer: D. 60 psi A visible warning, usually a red light and sometimes a buzzer or wig wag, must come on before pressure drops below 60 psi (or half the cut-out pressure on older vehicles).
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Answer: B. 20 to 45 psi Spring brakes come fully on somewhere between 20 and 45 psi, typically 20 to 30 psi. Do not wait for them: stop safely as soon as the low-air warning comes on.
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Answer: C. Within 45 seconds The manual's check is 85 to 100 psi within 45 seconds for dual systems. Trucks with larger tanks may take longer, so check the manufacturer's specs.
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Answer: A. Less than 3 psi in one minute With the brakes released, the loss should be under 2 psi per minute for a single vehicle and under 3 psi per minute for a combination.
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Answer: D. More than 3 psi With the brakes applied, a drop of more than 3 psi per minute (single) or more than 4 psi per minute (combination) means a leak to fix before driving.
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Answer: B. If it moves more than about one inch where the pushrod attaches More than about one inch of movement at the pushrod means the brake is likely out of adjustment. Out-of-adjustment brakes are the most common roadside inspection problem.
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Answer: A. To remove water and oil that can damage the system or freeze Compressed air carries water and compressor oil that collect in the tanks. Water can freeze in cold weather and cause brake failure, so drain the tanks unless they drain automatically.
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Answer: C. At 40 mph, brake firmly to about 35 mph, then release, and repeat Brake just hard enough for a definite slowdown to about 5 mph below your safe speed, then release. Each application should last about three seconds.
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Answer: D. The delay, half a second or more, for air to flow to the brakes after you press the pedal Hydraulic brakes act almost instantly, but air takes half a second or more to reach the brakes. That adds distance to your total stop.
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Answer: A. Stop and safely park as soon as possible, while you still have air to brake There may be a leak. It is much safer to stop with the foot brake while there is still air than to let the spring brakes come on by themselves.
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Answer: B. When the brakes are very hot or very wet in freezing weather Very hot brakes can be damaged by setting the parking brakes, and very wet brakes can freeze in place. Chock the wheels and let hot brakes cool first.
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Answer: C. As soon as the wheels start rolling again Apply fully, release when the wheels lock, and reapply once they roll again. It can take up to a second for the wheels to start rolling.
About this test
You need to pass the air brakes knowledge test to drive a CDL vehicle with air brakes without a restriction. Under 49 CFR 383.135, anyone who scores under 80 percent on the air brake test and still gets a CLP or CDL will have an air brake restriction on the license. The test covers system parts (compressor, governor, tanks, safety valve, spring brakes), dual air systems, the inspection checks and how to use air brakes on the road.
These are practice questions, not the official test. The numbers here come from the model CDL manual used by most states, and many say "about" or "check the manufacturer's specifications" because real trucks vary. Study the air brakes section of your state's CDL manual before test day.
More CDL practice tests
- New York State Commercial Driver's Manual (CDL-10), Section 5: Air Brakes
- Rhode Island DMV Commercial Driver's License Manual, Section 5: Air Brakes
- 49 CFR 383.135: Passing knowledge and skills tests (eCFR)
This page is general information, not legal or tax advice. Rules change; confirm with FMCSA, the IRS or your state before you file.
Questions carriers ask
What happens if I fail the air brakes test?
Under 49 CFR 383.135, if you score below 80 percent on the air brake knowledge test and are still issued a CLP or CDL, it will carry an air brake restriction. That means you cannot drive a CMV with air brakes until you remove it.
Do I need the air brakes test for every CDL?
You need it if you will drive a CDL vehicle equipped with air brakes and want to do so without a restriction. Most tractor-trailers and many heavy trucks and buses use air brakes.
Are the psi numbers the same on every truck?
No. The manual gives typical values, such as governor cut-out around 125 psi and cut-in around 100 psi, and tells you to check the manufacturer's specifications. The low-air warning must come on before pressure drops below 60 psi.