California CDL — Air Brakes practice
California CDL — Air Brakes: Safe Driving Practices practice test
Following distance, scanning, blind spots, using your lights, and the habits that prevent crashes and keep you in control.
Content reviewed against official driver handbooks · August 12, 2026
20 questions · pass with 16 correct. You get instant feedback and an explanation after every answer.
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Study questions with answers
12 sample Safe Driving Practices questions with the correct answer, a short explanation, and the official handbook reference. Read through them, then take the quiz above.
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1. When testing air pressure buildup in a vehicle with a dual air system, pressure should build from 85 to 100 psi in how long (with the engine at operating rpm)?
Correct answer: Within 45 seconds
In dual air systems, pressure should build from 85 to 100 psi within 45 seconds with the engine at operating rpm. If pressure builds more slowly, something is wrong and it must be fixed before driving.
2. Some older vehicles have a front brake limiting valve with "normal" and "slippery" positions. What does the handbook say about using this control?
Correct answer: Leave it in the "normal" position, because front-wheel braking is good under all conditions
Limiting valves were meant to reduce the chance of front-wheel skids on slippery surfaces, but tests have shown front-wheel braking is good under all conditions. The valve should be left in the normal position, because the slippery setting reduces the vehicle's stopping power.
3. Which type of foundation brake is found at the wheels of most heavy vehicles that use air brakes?
Correct answer: S-cam drum brakes
The s-cam drum brake is the most common type of foundation brake on air-braked vehicles. It is named for the s-shaped camshaft that pushes the brake shoes against the inside of the drum.
4. How do you test the service brakes during the final air brake check?
Correct answer: Roll forward slowly at about 5 mph and press the brake pedal firmly, watching for pulling or a delayed stop
Once air pressure is normal and the parking brake is released, ease the vehicle forward at roughly 5 mph and brake firmly. Pulling to one side, an odd feel, or a slow response reveals problems you might not otherwise discover until you depend on the brakes in traffic.
5. If your air tanks have manually operated drain valves, why must you drain them at the end of every driving day?
Correct answer: To remove water and compressor oil that could freeze and cause brake failure
Compressed air usually carries some water and compressor oil, which collect at the bottom of the tanks. In cold weather that water can freeze and make the brakes fail, so tanks with manual drain valves must be drained completely by you after every driving day.
6. The air compressor governor controls when the compressor pumps air into the storage tanks. At about what pressure does the governor stop the compressor from pumping (the "cut-out" pressure)?
Correct answer: About 125 psi
When air tank pressure rises to around 125 psi, the governor cuts out the compressor so it stops pumping air. This keeps the system from over-pressurizing during normal operation.
7. The first air tank the compressor fills is fitted with a safety relief valve. At about what pressure does this valve usually open?
Correct answer: 150 psi
The safety valve keeps the tanks and the rest of the system from being over-pressurized and is typically set to open at about 150 psi. If it ever lets air out, something is wrong, so have a mechanic fix the fault.
8. The low air pressure warning signal on a vehicle with air brakes must come on before air pressure in the tanks falls below what level?
Correct answer: 55 psi
A low air pressure warning is required on vehicles with air brakes and must activate before pressure drops below 55 psi in the air tank. It may be a red light, a buzzer, or a wig wag device that drops into your view.
9. During the walk-around inspection, when is a crack in a brake drum too long to be acceptable?
Correct answer: When it is longer than one-half the width of the friction area
Brake drums or discs fail the inspection if a crack runs more than half the width of the friction area.
10. The safety relief valve on the first air tank opens and releases air. What does that tell you?
Correct answer: Something is wrong, and a mechanic should fix the fault
The safety valve is there to keep excess pressure from damaging the tank or other parts of the air system. It should stay closed in normal operation, so if it does release air there is a fault that a mechanic needs to repair.
11. At about what tank pressure does the governor "cut in" and make the air compressor start pumping again?
Correct answer: About 100 psi
When tank pressure falls to around 100 psi, the governor cuts in and the compressor resumes pumping air. The compressor then builds pressure back up to the cut-out level of about 125 psi.
12. Dual air brake systems are used on most heavy-duty vehicles. What safety advantage does a dual system give you?
Correct answer: Because the two systems are separate, losing air in one still leaves the brakes on the other system working
A dual system is two separate air brake systems, each with its own tanks and lines, sharing one set of brake controls; one usually works the rear-axle brakes and the other the front. If one system's pressure gets very low, the brakes on the front or the rear won't be working fully and stopping takes longer, so get the vehicle stopped safely and have the system repaired.
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California Practice Test Pack — 2026 Printable Edition
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Our own printable edition — nobody paid us to show it to you, and everything in it is free to read here too.