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Air Brakes Endorsement CDL Practice Test Part A

CDL Test Practice Exams & CDL Training Materials

Taking the CDL test will be much easier after going through our practice CDL tests here online. This is the Air Brakes portion of the CDL written practice exam. These questions are very similar to the ones you'll find on the actual CDL written test you'll be taking to get your CDL license. Follow along with the test questions below and read through the explanations that follow each CDL test question to get some better insights into the CDL exam and the trucking industry itself.

1. Why must air tanks be drained?

A. To drain all the air out of the tank.
B. To drain moisture and oil out of the tank.
C. To keep anyone from moving the vehicle.


Answer

B. To drain moisture and oil out of the tank.



2. What is a supply pressure gauge used for?

A. To tell you how much diesel is in the vehicle.
B. To tell you how much water is in the air tank.
C. To tell you how much air is in the air tanks.


Answer

C. To tell you how much air is in the air tanks.

Tractor trailers generally have two separate air systems - one for the tractor and one for the trailer. You will find two air gauges on the dashboard in most trucks. Generally speaking, the trailer air tank operates the trailer brake system and the tractor air tank operates the air-splitter on the shifter, the windshied wipers (yes, they are usually air-operated), the tractor brakes, and whatever other systems run on air.

It's possible to have an air leak in one system which will not affect the other. Say for instance you have a blown-out air chamber on the trailer brakes. You'll likely notice that the trailer air system is struggling to maintain its air pressure while the tractor air pressure remains stable. Both gauges are hooked up to the warning system, so you will be warned of a loss of air pressure in either system. Both systems run off the same air compressor, so if your air compressor malfunctions you will not be able to build or maintain pressure in either system.

3. All vehicles with air brakes must have a low air pressure warning signal.

A. True
B. False


Answer

A. Totally true

There is always a simple warning system built into all commercial vehicles with air brakes which usually consists of both a warning light on the dash and a buzzer sound.

4. What are spring brakes?

A. Brakes used for parking.
B. If the airbrakes leak down, springs press the brakes on to stop the vehicle.
C. The spring on the brake pedal.


Answer

B. If the airbrakes leak down, springs press the brakes on to stop the vehicle.

Air brake systems have one huge safety advantage over hydraulic brakes. By default, air brake systems are locked down. It actually takes air pressure to release the brakes. Air pressure is also used to apply the brakes. Air brake systems on commercial vehicles are designed to lock down if you lose air pressure. If you have a failure in the braking system or in the air compressor and you can't maintain air pressure, the brakes will lock down the wheels. So in a tractor trailer, when you "apply your parking brakes", all you are really doing is releasing all of the air from the braking system, allowing the spring brake to apply the brakes and lock down the wheels.

With hydraulic braking systems, the brakes are released by default. It takes hydraulic pressure to apply the brakes. If you lose hydraulic pressure through a failure of some sort in the braking system, you will lose your brakes entirely.

5. Front wheel brakes are good under all conditions.

A. True
B. False


Answer

A. True

6. When you apply your brakes, the air brake system works with an electric switch, by air pressure, to turn on the brake lights.

A. True
B. False


Answer

A. True

First of all, who is coming up with these questions?

Ok, anyhow - yes, the brake lights are operated by an electric switch which is activated by air pressure. Glad we know that.

7. What do air brakes use to make the brakes work?

A. Compressed gas
B. Compressed oil
C. Compressed air


Answer

C. Compressed air

Compressed air is used to overcome the spring pressure that locks down the brakes in order to release them in the first place. Then, air pressure is used once again to apply the brakes.

8. Air brakes are really three different braking systems: service brake, parking brake and emergency brake.

A. True
B. False


Answer

A. True

The service brake is when you apply foot pressure to the pedal to stop the vehicle.

The parking brake is applied when you release all air from the braking system by pulling the knob on the dash.

The emergency braking systems allows the knob on the dash to pop out when the air pressure gets too low, locking down the brakes.

9. The service brake applies and releases:

A. The parking brake when you use the parking brake control.
B. The brake when you use the brake pedal during normal driving.
C. In the event of an emergency.


Answer

B. The brake when you use the brake pedal during normal driving.

The service brake is when you apply foot pressure to the pedal to stop the vehicle.

The parking brake is applied when you release all air from the braking system by pulling the knob on the dash.

The emergency braking systems allows the knob on the dash to pop out when the air pressure gets too low, locking down the brakes.

10. The parking brake applies and releases:

A. The brake pedal during normal driving.
B. In the event of an emergency.
C. The parking brakes when you use the parking brake control.


Answer

C. The parking brakes when you use the parking brake control.

The service brake is when you apply foot pressure to the pedal to stop the vehicle.

The parking brake is applied when you release all air from the braking system by pulling the knob on the dash.

The emergency braking systems allows the knob on the dash to pop out when the air pressure gets too low, locking down the brakes.

11. The emergency brake system:

A. Uses parts of the service and parking brake system to stop the vehicle in the event of a brake system failure.
B. Uses the service brake to stop the vehicle in the event of a brake system failure.
C. Uses the parking brake to stop the vehicle in the event of a brake system failure.


Answer

A. Uses parts of the service and parking brake system to stop the vehicle in the event of a brake system failure.

The service brake is when you apply foot pressure to the pedal to stop the vehicle.

The parking brake is applied when you release all air from the braking system by pulling the knob on the dash.

The emergency braking systems allows the knob on the dash to pop out when the air pressure gets too low, locking down the brakes.

12. The governor controls when the air compressor will pump into the air storage tanks.

A. True
B. False


Answer

A. True

The governor is a device that maintains the proper air pressure in the air tanks of a tractor trailer. When the pressure is too low, the governor will allow air coming out of the compressor to enter the air tanks. When the air pressure rises to the maximum pressure allowed in the system, the governor prevents more air from entering the air tanks.

13. The air storage tanks are used to hold compressed air.

A. True
B. False


Answer

A. True

The storage tanks hold the air that runs any of the systems on the truck that operate by air pressure, including the brakes, often times the windshied wipers, the air-splitter on the shifter, and any other systems there may be. You can not run these systems directly off of the air coming out of the air compressor because you wouldn't have the volume of air needed to operate these systems immediately available when needed, and you wouldn't have any reserve capacity available in case of an air compressor failure.

14. When you put on the brakes by pushing down the brake pedal:

A. The air pressure is reduced to the brake pads.
B. Air pressure is applied to the brake pads.
C. Compressed air is let go out of the system.


Answer

B. Air pressure is applied to the brake pads.

When you release the parking brakes, air pressure works against the spring brakes to release the brakes. When you use the brake pedal, more air pressure is put into the system to assist the spring brake in stopping the truck. Then, when you release the brake pedal, that additional air is once again released from the system, releasing the brakes.

15. The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. It is called the S-cam because:

A. It is shaped like the letter Z.
B. It is shaped like the letter N.
C. It is shaped like the letter S.


Answer

C. It is shaped like the letter S.

Believe it or not people, they really do have some questions that are this easy on the test.

When you apply the brakes with the brake pedal, air pressure pushes against a bladder that is attached to a pushrod. The pushrod turns the S-cam which forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam is forced to return to its original position, releasing the brakes once again.

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