Air Brakes Practice Test
Not an endorsement — a restriction you remove. Fail or skip it and an L restriction goes on the licence, barring you from any commercial vehicle with full air brakes.
This is not an endorsement. Passing it removes the L restriction — skip or fail it and your licence is stamped "no full air brakes", which bars you from most commercial vehicles on the road.
Do you need this test?
Required for Class A, Class B. Everyone takes the general knowledge test as well.
Passing this test is what removes the L restriction from your licence. See all restriction codes.
Practice now
Study the material, not just the questions
Master the system that decides whether a loaded truck stops.
Open the Air Brakes moduleThe other tests you'll need
- General Knowledge — 50 questions · 80% to pass
- Combination Vehicles — 20 questions · 80% to pass
- Hazardous Materials — 30 questions · 80% to pass
- Tank Vehicles — 20 questions · 80% to pass
- Doubles and Triples — 20 questions · 80% to pass
- Passenger Transport — 20 questions · 80% to pass
- School Bus — 20 questions · 80% to pass
- LCV Doubles — 30 questions · 80% to pass
- LCV Triples — 30 questions · 80% to pass
See every CDL knowledge test →
More CDL guides
See every CDL knowledge test and guide →
Sample Air Brakes questions
At what PSI does the low air pressure warning usually activate?
- A. 40 PSI
- B. 60 PSI
- C. 50 PSI
- D. 70 PSI
Show answer
B. 60 PSI — The low air pressure warning signal must come on before air pressure drops below 60 PSI.
Universal rule · checked against FMCSA Commercial Driver License Manual — Section 5: Air Brakes · verified 2026-05-17
Air brake equipped vehicles must have:
- A. Manual parking brake only
- B. Hydraulic backup brakes for use if air is lost
- C. An air pressure gauge in the driver's view
- D. Electric brake assist
Show answer
C. An air pressure gauge in the driver's view — All vehicles with air brakes must have an air pressure gauge visible to the driver to monitor system pressure.
Universal rule · checked against FMCSA Commercial Driver License Manual — Section 5: Air Brakes · verified 2026-05-17
The air compressor is connected to the engine through:
- A. Electrical wires
- B. Air hoses
- C. Hydraulic lines
- D. Gears or a V-belt
Show answer
D. Gears or a V-belt — The air compressor is driven by the engine through gears or a V-belt and pumps air into the air storage tanks.
Universal rule · checked against FMCSA Commercial Driver License Manual — Section 5: Air Brakes · verified 2026-05-17
Air brakes use _____ to make the brakes work:
- A. Compressed air
- B. Hydraulic fluid
- C. Electricity
- D. Vacuum pressure
Show answer
A. Compressed air — Air brakes use compressed air to push brake pads against brake drums or discs to slow/stop the vehicle.
Universal rule · checked against FMCSA Commercial Driver License Manual — Section 5: Air Brakes · verified 2026-05-17
Why drain water from compressed air tanks?
- A. Water causes rust in the fuel tank
- B. Water increases fuel consumption
- C. Water makes the engine run rough
- D. Water can freeze in cold weather and cause brake failure
Show answer
D. Water can freeze in cold weather and cause brake failure — Compressed air contains water vapor which condenses in tanks. Water can freeze and block air lines, causing brake failure.
Universal rule · checked against FMCSA Commercial Driver License Manual — Section 5: Air Brakes · verified 2026-05-17
Frequently asked questions
How many questions are on the Air Brakes test?
25 multiple-choice questions, and you need 80% correct — so you can miss 5. The federal standard is consistent across nearly every state.
Is this Air Brakes practice test free?
Yes. Every practice test here and everything in the DMV Prep app is free — no ads, no paywall, no account.
Where do these questions come from?
The FMCSA Commercial Driver's License Manual and 49 CFR. Every question carries its citation, which you can open from the answer explanation.
