CDL Commercial License Prep practice questions and answers 2026. Tap an option to test yourself — you'll see the correct answer and a plain-English explanation for every question. Free, no login.
Q21What is the minimum air pressure required in the tanks for safe operation of air brakes?
✓ Correct answer: D. 90 psi
Commercial vehicles with air brakes must maintain at least 90 psi (pounds per square inch) in the air tanks for safe operation. Below this pressure, the braking capability becomes compromised.
Q22At what pressure should the low air warning device activate?
✓ Correct answer: C. 55-60 psi
Federal regulations require the low air warning device to activate when air pressure drops below 60 psi, giving the driver time to stop safely before brake function is compromised.
Q23What is the maximum allowable air pressure loss in a single vehicle during the static leakage test?
✓ Correct answer: B. 2 psi per minute
For a single vehicle with air brakes, the maximum allowable air pressure loss during a static leakage test (brake pedal not applied) is 2 psi in one minute.
Q24What is the maximum allowable air pressure loss in a combination vehicle during the applied leakage test?
✓ Correct answer: A. 3 psi per minute
For a combination vehicle with air brakes, the maximum allowable air pressure loss during an applied leakage test (brake pedal applied) is 3 psi in one minute.
Q25What is the primary purpose of the air brake system's governor?
✓ Correct answer: D. To control when the air compressor pumps air
The governor controls when the air compressor pumps air into the storage tanks, typically cutting in around 100 psi and cutting out around 125 psi, maintaining proper system pressure.
Q26How often should air tanks be drained to remove moisture and oil?
✓ Correct answer: C. Daily
Air tanks should be drained daily to prevent moisture and oil accumulation that can freeze in cold weather or contaminate the system, leading to brake failure.
Q27What happens to spring brakes when air pressure is lost?
✓ Correct answer: B. They automatically apply
Spring brakes automatically apply when air pressure drops too low, serving as an emergency braking system when normal air pressure is lost.
Q28What component in the air brake system helps remove moisture?
✓ Correct answer: A. Air dryer
The air dryer removes moisture from compressed air before it enters the system, helping prevent corrosion and freezing issues in the air tanks and lines.
Q29How long should it take for air pressure to build from 85 to 100 psi at idle speed?
✓ Correct answer: D. 45 seconds or less
At idle speed, it should take no more than 45 seconds for air pressure to build from 85 to 100 psi. This tests the air compressor's ability to replenish air supply.
Q30What is the purpose of the supply pressure gauge in an air brake system?
✓ Correct answer: C. To show the pressure available in the air tanks
The supply pressure gauge shows the pressure available in the air tanks, allowing the driver to monitor if sufficient air pressure is available for safe braking.
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