Free CDL Air Brakes Practice Test 2026
The Air Brakes test is required to operate any CMV equipped with air brakes. It covers air brake system parts, dual air systems, inspecting air brakes, and using them safely on grades and in emergencies.
In an emergency stop in a vehicle WITHOUT ABS, which braking methods should you use?
About the CDL Air Brakes Test
Air brakes stop most commercial vehicles, and they behave nothing like the hydraulic brakes on a car. This test covers how a dual air-brake system builds and stores pressure, why you test the low-pressure warning and check for leaks, and how to avoid brake fade by using controlled braking on long downgrades instead of riding the pedal. One detail trips up many drivers: if you skip or fail the air-brakes test, your CDL carries an 'L' restriction that bars you from operating any air-brake-equipped vehicle.
What's on the CDL Air Brakes knowledge test
The Air Brakes knowledge test is written from Section 5 (Air Brakes) of the national CDL manual, the text every state's CDL manual follows. You take it only if you need the AB endorsement or drive a vehicle that requires it.
Federal rules fix the pass mark at 80 percent for every CDL knowledge test, but leave the number of questions to each state, so check your own state's CDL manual for the exact count. This practice test uses all 20 questions in our Air Brakes bank, each checked against the manual page shown with it.
- Source manual
- National CDL Manual, Section 5 (Air Brakes)
- Number of questions
- Set by each state: federal rules require a set number of questions, with a prescribed number from each knowledge area (49 CFR 383.133)
Manual sections this test is drawn from
- Section 5: Air Brakes - The Parts of an Air Brake System, Dual Air Brake, Inspecting Air Brake Systems, Using Air Brakes
Sample questions from this test, with answers
Want to test yourself first? Take the test above before reading these. Every answer below is checked against the source shown.
1. What should you do if the low air pressure warning comes on while driving?
- A. Keep driving to the next repair shop
- B. Pump the brakes to build pressure back
- C. Speed up to recharge the air tanks
- D. Stop and safely park as soon as you can (correct)
Why: The warning may mean an air leak. You can brake under control only while air remains, and the spring brakes come on at about 20 to 45 psi, so stop while you can still use the foot brake.
Source: National CDL Manual, p.5-9 (PDF p.83), 5.4.7 Low Air Pressure
2. What does the application pressure gauge show?
- A. How much air is stored in the tanks
- B. How much air you are applying to the brakes (correct)
- C. The oil pressure in the engine while it is running
- D. The pressure inside each of the tires
Why: This gauge, which not every vehicle has, shows the air pressure going to the brakes. Needing more and more pressure to hold the same speed means the brakes are fading: slow down and use a lower gear.
Source: National CDL Manual, p.5-2 (PDF p.76), 5.1.10 Application Pressure Gauge
3. In a dual air system at operating rpm, pressure should build from 85 to 100 psi within:
- A. 45 seconds (correct)
- B. 3 minutes
- C. 10 minutes
- D. 15 seconds
Why: In dual air systems the pressure should rise from 85 to 100 psi within 45 seconds at operating rpm. Older single systems typically go from 50 to 90 psi within 3 minutes at idle.
Source: National CDL Manual, p.5-7 (PDF p.81), 5.3.3 Check Rate of Air Pressure Buildup
4. In the air leakage test (system fully charged, engine off, parking brake released, no brakes applied), a single vehicle should lose less than:
- A. 4 psi in one minute
- B. 6 psi in one minute
- C. 10 psi in one minute
- D. 2 psi in one minute (correct)
Why: With the brakes released, the loss should be under 2 psi per minute for a single vehicle and under 3 psi for a combination. A faster drop means a leak that must be fixed before you drive.
Source: National CDL Manual, p.5-6 (PDF p.80), 5.3.3 Step 7 Final Air Brake Check (Test Air Leakage Rate)
5. What three braking systems make up an air brake system?
- A. Front, rear, and trailer brakes
- B. Hydraulic, spring, and engine brakes
- C. Service, parking, and emergency brakes (correct)
- D. Primary, secondary, and backup brakes
Why: An air brake setup is really three systems in one: the service brake you work with the pedal, the parking brake, and the emergency brake, which uses parts of the other two to stop the vehicle if the system fails.
Source: National CDL Manual, p.5-1 (PDF p.75), Section 5 Air Brakes
6. Before tank pressure falls below what level must the low air warning come on?
- A. 60 psi
- B. 55 psi (correct)
- C. 100 psi
- D. 20 psi
Why: A warning you can see, usually a red light and sometimes a buzzer, must come on before pressure drops below 55 psi (on older vehicles, half the governor cut-out pressure). On large buses it often comes on at 80-85 psi.
Source: National CDL Manual, p.5-3 (PDF p.77), 5.1.11 Low Air Pressure Warning
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