CDL Air Brakes Practice Test
Covers the air brake system, proper use, and inspection. Skipping it adds an "air brake" restriction to your license.
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About the CDL Air Brakes test
The CDL Air Brakes test covers how air brake systems work, how to inspect them, and how to use them safely. You must pass it to drive any commercial vehicle with air brakes; skip it and your license carries an air brake restriction that bars you from those vehicles. The material comes from Section 5 of the federal Commercial Driver License Manual, so it applies in every state.
What the Air Brakes test covers
- System parts: the air compressor and governor, reservoirs, the foot (treadle) valve, brake chambers, slack adjusters, and S-cam or disc brakes.
- Dual air brake systems: separate primary and secondary circuits, and what to do if one loses pressure.
- Inspecting air brakes: the three-part air brake check, build-up time, the low-air warning, and air-loss (leakage) limits.
- Using the brakes: normal stops, controlled and stab braking, and managing brake fade on long downgrades.
- Spring and parking brakes: how they apply automatically when air pressure drops too low, and the tractor protection valve.
Common mistakes to avoid
The most-missed questions involve the exact pressure numbers and the leakage checks: the low-air warning must come on before pressure drops below 60 psi, the safety valve opens around 150 psi, and spring brakes apply automatically somewhere around 20 to 45 psi. People also confuse the air-loss limits during the static and applied leakage tests, and many forget that air brakes have a brake lag (about half a second of delay before they grip) that adds stopping distance. Learn the numbers and the three-part check in order.
How to study for it
Memorize the key pressure thresholds and the order of the three-part air brake check, then run the full practice test until the numbers are automatic. Switch to the exam simulator to lock them in under time. Air Brakes pairs naturally with the Combination Vehicles test, since most drivers who need one need the other.
Other CDL tests
General Knowledge
Required for every CDL applicant (Class A, B, and C).
Combination Vehicles
Required for every Class A (tractor-trailer) applicant.
HazMat (H)
Drivers hauling placarded hazardous materials.
Passenger (P)
Bus, transit, and shuttle drivers.
School Bus (S)
School bus drivers (requires the Passenger endorsement first).
Tanker (N)
Drivers of liquid or gas tank vehicles.
CDL Air Brakes test FAQ
How many questions are on the CDL Air Brakes test?
Most states use a 25-question Air Brakes test and require about 80% correct to pass. Our full practice mode and exam simulator use the same 25-question length.
What happens if I skip the Air Brakes test?
Your CDL gets an air brake restriction, which means you may not legally drive a commercial vehicle equipped with air brakes. To remove it, you pass the Air Brakes knowledge test and a skills test in a vehicle with air brakes.
At what pressure does the low-air warning come on?
The low air pressure warning must activate before the system pressure falls below 60 psi. If you keep losing air, the spring (parking) brakes will apply automatically, typically somewhere between about 20 and 45 psi.
Is the Air Brakes test the same in every state?
Yes, in substance. The content is federal, from Section 5 of the model CDL manual, so it is nearly identical across states, with only minor local differences in scheduling and fees.