PermitPal
CDL Air Brakes Practice Test
Federally standardized — same test in every state · Updated July 12, 2026
The Air Brakes knowledge test is 25 questions at 80% (miss up to 5) in nearly every state, because the material comes from the same federal model (49 CFR 383 / AAMVA). Fail it and your CDL carries the L restriction (no air-brake vehicles), so almost everyone takes it.
Answer each question to see the explanation. Your score is tallied against the real passing bar at the end.
1. Air brakes are really three separate braking systems. They are the:
Show answer & explanation
Correct answer: Service brake, parking brake, and emergency brake systems
The service brakes stop you in normal driving, the parking brakes hold the vehicle when stopped, and the emergency brake system uses parts of both to stop you if the service brakes fail.
2. What is the job of the air compressor?
Show answer & explanation
Correct answer: It pumps air into the air storage tanks
The compressor is driven by the engine and pumps air into the storage tanks (reservoirs). The stored air is what actually operates the brakes.
3. If the air compressor has its own oil supply, you should:
Show answer & explanation
Correct answer: Check the oil level before driving
A compressor with its own oil supply must be checked before driving, just like the engine. Running it low on oil can wreck the compressor and leave you without air.
4. The governor stops the compressor from pumping air at the "cut-out" level, which is around:
Show answer & explanation
Correct answer: 125 psi
When tank pressure rises to the cut-out level of about 125 psi, the governor stops the compressor. This keeps the system from building unlimited pressure.
5. The governor lets the compressor start pumping again at the "cut-in" level, which is around:
Show answer & explanation
Correct answer: 100 psi
When tank pressure falls to about 100 psi, the governor lets the compressor cut back in and rebuild the air supply. It cycles between roughly 100 and 125 psi.
6. Why must the air tanks be drained?
Show answer & explanation
Correct answer: Water and compressor oil collect in them and can cause brake failure
Compressed air carries water and oil that settle in the tank bottom. In cold weather the water can freeze and knock out the brakes, so the tanks must be drained.
7. If your air tanks have manually operated drains, how often should you drain them?
Show answer & explanation
Correct answer: Completely, at the end of each day of driving
Manual drain valves must be opened at the end of each driving day to clear out the water and oil. Automatic drains do it for you, but should still be checked.
8. The alcohol evaporator is used to:
Show answer & explanation
Correct answer: Reduce the risk of ice in the air brake valves in cold weather
The evaporator puts a little alcohol into the system to help keep ice from forming in the valves during cold weather. You still have to drain the tanks daily to remove water and oil.
9. The safety relief valve in the first air tank is usually set to open at:
Show answer & explanation
Correct answer: 150 psi
The safety valve protects the system from too much pressure and normally opens at 150 psi. If it ever lets air out, something is wrong and it needs a mechanic.
10. While driving you notice the safety valve releasing air. This means:
Show answer & explanation
Correct answer: Something is wrong and the fault should be fixed by a mechanic
The safety valve only vents when pressure climbs too high, which points to a governor or compressor problem. Releasing air is a warning sign, not normal operation.
11. The brake pedal in an air brake vehicle is also called the:
Show answer & explanation
Correct answer: Foot valve or treadle valve
Pushing the foot valve (treadle valve) down harder applies more air pressure to the brakes; letting up reduces the pressure and releases them.
12. Pressing and releasing the brake pedal unnecessarily can be dangerous because it:
Show answer & explanation
Correct answer: Lets air out faster than the compressor can replace it
Each release lets some compressed air escape. Fanning the pedal can drain the tanks faster than the compressor refills them, and if pressure gets too low the brakes stop working.
13. The most common type of foundation brake is the:
Show answer & explanation
Correct answer: S-cam drum brake
Most air-brake vehicles use s-cam drum brakes. Wedge and disc brakes exist but are far less common.
14. When you push the brake pedal on an s-cam brake, the s-cam:
Show answer & explanation
Correct answer: Forces the brake shoes apart and presses them against the drum
Air enters the brake chamber, moves the slack adjuster, and twists the s-cam, which spreads the shoes against the inside of the drum. The friction slows the wheel.
15. Too much heat in the brake drums can:
Show answer & explanation
Correct answer: Make the brakes fade or stop working
Braking creates heat, and brakes are built to take a lot of it — but excessive heat causes brake fade, and too much heat can make the brakes stop working entirely.
📱 PermitPal in your browser — the full CDL bank with real-format mock exams for all three sections. Free to start, $29.99 once. Also on the App Store and Google Play.