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CDL Air Brake Knowledge Practice Questions
1. What is a dual air brake system?

A. A two brake system, one operates the front brakes and one operates the back brakes.
B. A two braking system, one for each side of the vehicle.
C. A two braking system, one for daytime and one for nighttime driving.

2. What are the slack adjusters?

A. It is the space between the back wheels.
B. It is an adjusting nut on the backside of the brake drum used for adjusting the brakes.
C. It is an adjusting nut under the cab to adjust the springs.

3. How can you check the slack adjuster?

A. Park on level ground, turn off parking brakes, pull hard on each slack adjuster. It shouldn't move more than one inch.
B. Park on level ground, turn off parking brake and press brake pedal.
C. Park on an incline and see if the vehicle rolls.

4. How can you test the low pressure warning signal?

A. While the engine is running, step on and off the brake pedal.
B. Shut off the engine and see if the air pressure leaks down.
C. Shut off the engine with enough air pressure, turn on the electrical power and step on and off the brake pedal until the low air pressure warning signal comes on.

5. What can you check to see if the spring brakes come on automatically?

A. Step on and off the brake, with the engine off, the parking brake knob should pop out when air pressure falls between 20-40 psi.
B. Get under the vehicle and pull on the spring brakes.
C. Turn on the engine and pump the brake pedal down to the floor.

6. What are the maximum leakage rates?

A. Less than four psi in one minute for single vehicles and less than eight psi in one minute for combination vehicles.
B. Less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles.
C. Less than six psi for all vehicles.

7. Should you ever leave your vehicle unattended without applying parking brakes or chocking the wheels?

A. Yes
B. No

8. If you are away from your vehicle only a short time, you don't need to use the parking brake.

A. True
B. False

9. How often should you drain air tanks?

A. Daily
B. Weekly
C. Monthly

10. The use of brakes on a long steep downgrade is only a supplement to the braking effect of the engine.

A. True
B. False

11. What factors can cause brakes to fade or fail?

A. Not pressing the brake pedal hard enough.
B. Over heating, low air pressure, and not relying on the engine braking effect.
C. Not taking your foot off the accelerator pedal.

12. Why should you be in the proper gear before starting down a hill?

A. So you only have to apply the brake just hard enough to feel a definite slowdown.
B. So you don't have to shift gears going downhill.
C. So you can go through the gears on the way down.

13. If oil and water collect in the air tanks what can happen to the brakes?

A. The brakes could heat up.
B. The brakes could fail.
C. The brakes could lock up.

14. What is the purpose of an alcohol evaporator?

A. So you don't have to drain the air tanks as often.
B. To have a drink available if you get thirsty.
C. To reduce the risk of ice in the brake valves and other parts in cold weather.

15. What is another name for controlled braking?

A. Steady braking
B. Squeeze braking
C. Hard braking

Answer Key
1. A
2. B
3. A
4. C
5. A
6. B
7. B
8. B
9. A
10. A
11. B
12. A
13. B
14. C
15. B

1. Combination vehicles are usually heavier, longer and require more driving skill than single commercial vehicles.

A. True
B. False

2. More than half of truck driver deaths in crashes are the result of truck rollovers. Trucks turn over easier when fully loaded and are:

A. Easy to get turned back right.
B. Five times more likely to roll over in a crash than empty rigs.
C. Ten times more likely to roll over in a crash than empty rigs.

3. The following two things will help prevent rollovers.

A. Keep the cargo close to the front and drive slowly around turns.
B. Keep the cargo as close to the ground as possible and drive slowly around turns.
C. Keep the cargo to the back of the rig and drive the speed limit.

4. When you turn suddenly while pulling doubles, which trailer is most likely to turn over?

A. The rear trailer is twice as likely to turn over as the tractor.
B. The front trailer is twice as likely to turn over as the tractor.
C. Each trailer is equally likely to turn over as is the tractor.

5. Why should you not use the trailer hand brake to straighten out a jackknifing trailer?

A. Because the brake handle is too hard to reach.
B. Because the brakes on the trailer wheels cause the skid in the first place.
C. Because the brakes on the trailer wheels will not respond to the hand brake.

6. What is off tracking?

A. When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels.
B. When a vehicle goes around a corner, and the driver pulls off the road.
C. When a vehicle goes around a curve and turns back the other way.

7. What is a "trailer jackknife?"

A. When the tires on a trailer blow out.
B. When the wheels of a trailer lock up and the trailer swings around.
C. When the wheels of a trailer lock up and the trailer pulls the tractor to a stop.

8. Should you swing wide to the left before starting a 90 degree turn?

A. Yes
B. No

9. Why should you not use the trailer hand valve while driving?

A. Because it won't work as well as the foot brake.
B. Because you should use the parking brake.
C. Because of the danger of making the trailer skid.

10. Describe what the trailer air supply control does.

A. It is used to supply the trailer with air, shut the air off and put on the trailer emergency brakes.
B. It is a yellow, six sided knob used to control the tractor protection valve.
C. It is a device used to keep the trailer behind the tractor.

11. Describe what the service line is for:

A. To carry air to the parking brake to control the trailer brakes.
B. To carry air to a relay valve that allows the trailer brakes to be applied quickly.
C. To carry air to the cab of the rig to cool it.

12. What are other names for the Service Air Line?

A. Control line or signal line
B. Straight line or control line
C. Curved line or signal line

13. What is the emergency air line?

A. Air lines connecting the parking brake to the trailer brakes.
B. Air lines that control the emergency brakes of the tractor.
C. Air lines that control the emergency brakes on combination vehicles.

14. What is the emergency air line for?

A. To engage the front brakes of the tractor in case of an emergency.
B. To engage the trailer brakes in case of air pressure loss.
C. To have extra air pressure in case you need it.

15. Why should you use chocks when parking a trailer without spring brakes?

A. Because you don't want the trailer rolling down a hill.
B. Because if the air pressure leaks away or down, there will be no brakes.
C. Because some states require checking of trailer wheels.

Answer Key
1. A
2. C
3. B
4. A
5. B
6. A
7. B
8. B
9. C
10. A
11. B
12. A
13. C
14. B
15. B



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