South Dakota 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. 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.
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2. In real-world commercial driving, on a tractor-trailer with ABS, the trailer ABS warning lamp is mounted on the:
Explanation. A yellow lamp on the left side of the trailer indicates trailer ABS status. (Reinforcement variant — same underlying CDL Manual concept.)
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3. 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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4. 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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5. Brakes out of adjustment cause:
Explanation. Out-of-adjustment slack adjusters increase stroke and reduce braking force.
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6. On a tractor-trailer with ABS, the trailer ABS warning lamp is mounted on the:
Explanation. A yellow lamp on the left side of the trailer indicates trailer ABS status.
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7. Experienced commercial drivers know that modulating control valves let the driver:
Explanation. Modulating valves provide proportional braking, more pressure with more pedal force. (Reinforcement variant — same underlying CDL Manual concept.)
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8. Per federal regulations, 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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9. Experienced commercial drivers know 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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10. Experienced commercial drivers know that 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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11. Brake lag is:
Explanation. Air takes time to travel through lines; the typical lag is around 0.4 seconds.
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12. Application pressure gauge shows:
Explanation. Application pressure rises with pedal pressure and shows what is reaching the brake chambers.
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13. Most state DMV CDL handbooks state that the supply pressure gauge shows:
Explanation. The supply gauge displays reservoir pressure available for braking. (Reinforcement variant — same underlying CDL Manual concept.)
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14. Experienced commercial drivers know that 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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15. 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.
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16. 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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17. When taking the CDL knowledge exam, on a tractor-trailer with ABS, the trailer ABS warning lamp is mounted on the:
Explanation. A yellow lamp on the left side of the trailer indicates trailer ABS status. (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. 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.
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20. According to the FMCSA CDL Manual, 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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21. Most state DMV CDL handbooks state 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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22. 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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23. Modulating control valves let the driver:
Explanation. Modulating valves provide proportional braking, more pressure with more pedal force.
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24. Hydraulic brake systems use:
Explanation. Hydraulic brakes use brake fluid; CDL air-brake exam still requires you to know the difference.
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25. When the ABS warning lamp comes on while driving, you should:
Explanation. Service brakes still operate; have ABS serviced as soon as possible.
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 South Dakota-specific edition uses the same federal source material every U.S. state adopts. The South Dakota DPS administers the actual exam at its service centers. Bring your CDL handbook for any last-minute reference; it’s free at every South Dakota DPS office.
Once you’re consistently scoring above 90% on this practice set, you’re well above the 80% required to pass the official South Dakota 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.