North Carolina Air Brakes practice test
25 questions · 80% to pass · Required to operate any vehicle with air brakes.
How to use this practice test
Read each question, click an answer, and the correct choice is highlighted with a short explanation referencing the underlying CDL Manual concept. Your live score appears at the top of the page. Refresh to reset.
This test runs entirely in your browser. Nothing is uploaded; no account is required. Pages refresh to a new randomized cut from the bank.
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1. Pumping the brake pedal in an air-braked vehicle:
Explanation. Pumping releases stored air; never pump air brakes during normal stops.
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2. According to the FMCSA CDL Manual, how long should it take an air system to build from 85 to 100 psi?
Explanation. A typical dual air system should build from 85 to 100 psi within 45 seconds at engine governed rpm; older single systems within 30 seconds. (Reinforcement variant — same underlying CDL Manual concept.)
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3. The supply pressure gauge shows:
Explanation. The supply gauge displays reservoir pressure available for braking.
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4. During a pre-trip inspection, application pressure gauge shows:
Explanation. Application pressure rises with pedal pressure and shows what is reaching the brake chambers. (Reinforcement variant — same underlying CDL Manual concept.)
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5. Air brake reservoirs should be drained:
Explanation. Air tanks must be drained daily to remove water and oil that accumulate from compression.
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6. When taking the CDL knowledge exam, when the ABS warning lamp comes on while driving, you should:
Explanation. Service brakes still operate; have ABS serviced as soon as possible. (Reinforcement variant — same underlying CDL Manual concept.)
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7. During a pre-trip inspection, the low-pressure warning device must activate before pressure drops below:
Explanation. The warning must come on before air drops below 60 psi. (Reinforcement variant — same underlying CDL Manual concept.)
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8. Per federal regulations, total stopping distance for an air-braked vehicle includes:
Explanation. Air-braked vehicles add brake lag distance because of the time air takes to apply. (Reinforcement variant — same underlying CDL Manual concept.)
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9. Per federal regulations, brakes out of adjustment cause:
Explanation. Out-of-adjustment slack adjusters increase stroke and reduce braking force. (Reinforcement variant — same underlying CDL Manual concept.)
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10. Most state DMV CDL handbooks state that hydraulic brake systems use:
Explanation. Hydraulic brakes use brake fluid; CDL air-brake exam still requires you to know the difference. (Reinforcement variant — same underlying CDL Manual concept.)
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11. Experienced commercial drivers know that application pressure gauge shows:
Explanation. Application pressure rises with pedal pressure and shows what is reaching the brake chambers. (Reinforcement variant — same underlying CDL Manual concept.)
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12. In real-world commercial driving, when the air compressor governor reaches the cut-out pressure, it:
Explanation. The governor stops compression at cut-out, usually around 125 psi, and resumes at cut-in. (Reinforcement variant — same underlying CDL Manual concept.)
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13. Bleeding tanks of air during inspection lets you check for:
Explanation. Pressure drop tests check the integrity of the air system.
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14. When taking the CDL knowledge exam, brake lag is:
Explanation. Air takes time to travel through lines; the typical lag is around 0.4 seconds. (Reinforcement variant — same underlying CDL Manual concept.)
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15. During a pre-trip inspection, hydraulic brake systems use:
Explanation. Hydraulic brakes use brake fluid; CDL air-brake exam still requires you to know the difference. (Reinforcement variant — same underlying CDL Manual concept.)
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16. Most state DMV CDL handbooks state that you should drain the wet tank:
Explanation. The supply (wet) tank collects most water and should be drained, with all other tanks, daily. (Reinforcement variant — same underlying CDL Manual concept.)
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17. Experienced commercial drivers know that a leaking air system should be tagged out of service if pressure loss exceeds:
Explanation. For a parked single vehicle, more than 2 psi/min loss is too much; for a combination, 3 psi/min. (Reinforcement variant — same underlying CDL Manual concept.)
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18. When operating a CMV in interstate commerce, air storage tanks must be drained:
Explanation. Compressed air carries water and oil that must be drained to prevent corrosion and freeze-up. (Reinforcement variant — same underlying CDL Manual concept.)
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19. When operating a CMV in interstate commerce, if the application pressure rises significantly during steady braking, it may mean:
Explanation. A pressure rise during steady application indicates the brakes are not responding properly. (Reinforcement variant — same underlying CDL Manual concept.)
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20. Spring brakes (parking/emergency) come on automatically when air pressure drops between:
Explanation. Spring brakes apply at approximately 20-45 psi as air pressure falls.
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21. Per federal regulations, air brake systems combine three braking systems:
Explanation. Service brakes for normal stops, parking brakes for parking, and emergency brakes that use parts of both. (Reinforcement variant — same underlying CDL Manual concept.)
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22. According to the FMCSA CDL Manual, bleeding tanks of air during inspection lets you check for:
Explanation. Pressure drop tests check the integrity of the air system. (Reinforcement variant — same underlying CDL Manual concept.)
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23. When taking the CDL knowledge exam, pumping the brake pedal in an air-braked vehicle:
Explanation. Pumping releases stored air; never pump air brakes during normal stops. (Reinforcement variant — same underlying CDL Manual concept.)
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24. Air storage tanks must be drained:
Explanation. Compressed air carries water and oil that must be drained to prevent corrosion and freeze-up.
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25. Experienced commercial drivers know that the supply pressure gauge shows:
Explanation. The supply gauge displays reservoir pressure available for braking. (Reinforcement variant — same underlying CDL Manual concept.)
About the Air Brakes exam
The Air Brakes endorsement covers the operation of compressed-air braking systems used on most heavy commercial vehicles. Topics include compressor and governor operation, supply and service tanks, brake chambers, slack adjusters, dual air systems, parking brakes, low-pressure warning devices, and the seven-step pre-trip air brake test.
This North Carolina-specific edition uses the same federal source material every U.S. state adopts. The North Carolina DMV administers the actual exam at its service centers. Bring your CDL handbook for any last-minute reference; it’s free at every North Carolina DMV office.
Once you’re consistently scoring above 90% on this practice set, you’re well above the 80% required to pass the official North Carolina exam. Drill the questions you miss most often, then take the test cold once a day for a week leading up to your appointment.
Tips that actually work
- Read every answer choice before clicking. CDL questions are famous for "best answer" wording where two choices look right.
- Don’t memorize question text — learn the underlying rule. The DMV reshuffles wording constantly.
- Keep a list of the questions you miss. Re-drill them in isolation until you can’t miss them.
- Practice in short, frequent sessions. Two 20-minute sessions per day beats one two-hour cram.
What happens after I pass?
Passing the knowledge test earns you a Commercial Learner’s Permit (CLP). You must hold the CLP for at least 14 days before you can take the road skills test, and you must drive with a CDL-holding instructor in the cab during that period. Once you pass the road skills test, your CDL is issued.