CDL Endorsements

CDL Endorsements Air Brakes Practice Questions

Air Brakes makes up 11 of every 50 questions on a Tigertest CDL Endorsements practice test. Here are 10 of the 44 questions on it, each with the answer and the reason.

Updated 2026-11-10 by the Tigertest team. Based on FMCSA CDL Manual sections 4, 5, 6, 8 and 9. HazMat also needs a TSA security threat assessment.

1. 1. Air brakes on most heavy vehicles are really three braking systems working together. Which three are they?

   • A.Foot, hand, and transmission brakes
   • B.Service, parking, and emergency brakes
   • C.Service, engine brake, and hydraulic backup brakes
   • D.Primary, retarder, and exhaust brakes

   Answer: B. The air brake system combines the service brake, the parking brake, and the emergency brake. Engine, exhaust, and retarder brakes are separate devices, not part of the air brake system.

2. 2. At about what pressure does the air compressor governor stop the compressor from pumping air into the tanks?

   • A.About 150 psi
   • B.About 60 psi
   • C.About 100 psi
   • D.About 125 psi

   Answer: D. The governor cuts the compressor out at around 125 psi. It cuts back in near 100 psi, 150 psi is the safety relief valve setting, and 60 psi is the low air warning level.

3. 3. When tank pressure falls, at about what pressure does the governor make the compressor start pumping again?

   • A.About 100 psi
   • B.About 45 psi
   • C.About 125 psi
   • D.About 80 psi

   Answer: A. The governor cuts the compressor back in at around 100 psi. The compressor stops (cuts out) at about 125 psi.

4. 4. You finish a day of driving a truck whose air tanks have manual drain valves. What does the CDL manual recommend?

   • A.Drain the tanks once a week during inspection
   • B.Leave them closed so pressure stays high overnight
   • C.Drain the air tanks at day's end
   • D.Drain only after the low air warning comes on

   Answer: C. Manually drained tanks should be drained at the end of each day of driving so water and oil do not build up. Waiting for a warning or a weekly check lets moisture collect and freeze.

5. 5. Why is it important to drain water and compressor oil out of the air storage tanks?

   • A.They make the governor cut out the compressor too early
   • B.They dilute the alcohol in the alcohol evaporator
   • C.They can freeze or foul parts and cause brake failure
   • D.They push tank pressure past the safety valve setting

   Answer: C. Compressed air usually contains water and some compressor oil, which collect in the tanks. Water can freeze in cold weather and the sludge can damage valves, causing brake failure.

6. 6. It is winter and your truck has an alcohol evaporator in the air system. What should you do with it each day in cold weather?

   • A.Check and fill the alcohol container
   • B.Shut it off to protect the compressor
   • C.Use it instead of draining the air tanks
   • D.Drain it so the alcohol cannot freeze

   Answer: A. An alcohol evaporator puts alcohol into the air system to reduce the risk of ice in brake valves; check and fill the container daily in cold weather. You still need to drain the air tanks regularly.

7. 7. The safety relief valve installed in the first air tank is usually set to open at what pressure?

   • A.200 psi
   • B.125 psi
   • C.100 psi
   • D.150 psi

   Answer: D. The safety valve protects the tank and system from too much pressure and usually opens at 150 psi. If it releases air, something is wrong and should be fixed by a mechanic.

8. 8. While stopped in heavy traffic, you keep pressing and releasing the brake pedal out of habit. What effect can this have?

   • A.It warms the drums so the brakes work better
   • B.It can use air faster than it is replaced
   • C.It builds extra pressure in the storage tanks
   • D.It keeps the slack adjusters properly adjusted

   Answer: B. Each release of the pedal lets air out of the system and lowers tank pressure. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it.

9. 9. Which type of foundation brake is the most common on heavy vehicles that use air brakes?

   • A.Hydraulic band brakes
   • B.S-cam drum brakes
   • C.Wedge brakes
   • D.Air disc brakes

   Answer: B. S-cam drum brakes are the most common foundation brakes on air-braked vehicles. Wedge and disc brakes exist but are less common, and band brakes are not air foundation brakes.

10. 10. Going down a long grade, you see that it takes more application pressure to hold the same speed. What does this mean?

    • A.The safety relief valve has just opened
    • B.The spring brakes are dragging on the drums
    • C.The governor has cut out the compressor
    • D.The brakes are starting to fade

    Answer: D. Needing more application pressure to hold the same speed means the brakes are fading. Slow down and use a lower gear.