TruckingTruth logo

Air Brakes Endorsement CDL Practice Test Part B

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. 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.


Answer

A. A two brake system, one operates the front brakes and one operates the back brakes.

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.

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.


Answer

B. It is an adjusting nut on the backside of the brake drum used for adjusting the brakes.

Modern tractor trailers generally have the slack adjuster on the pushrod. However, many tractors and trailers nowadays come with automatic slack adjusters. When functioning properly, the automatic slack adjusters do just as the name implies - they adjust the brakes automatically for the driver. With automatic slack adjusters, the driver will no longer be required to manually adjust the brakes.

In some states it is now illegal for anyone that is not a certified mechanic to adjust the brakes on a tractor trailer. Please speak with the trucking company you are working for to find out the type of brake system they are using and how they want you to maintain them. It is very likely that you will have automatic slack adjusters and will not be required to do anything to the braking system yourself. Even if your equipment does not come equipped with automatic slack adjusters, it is likely that your company will do all maintenance to the equipment and will not want their drivers touching anything.

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.


Answer

A. Park on level ground, turn off parking brakes, pull hard on each slack adjuster. It shouldn't move more than one inch.

Modern tractor trailers generally have the slack adjuster on the pushrod. However, many tractors and trailers nowadays come with automatic slack adjusters. When functioning properly, the automatic slack adjusters do just as the name implies - they adjust the brakes automatically for the driver. With automatic slack adjusters, the driver will no longer be required to manually adjust the brakes.

In some states it is now illegal for anyone that is not a certified mechanic to adjust the brakes on a tractor trailer. Please speak with the trucking company you are working for to find out the type of brake system they are using and how they want you to maintain them. It is very likely that you will have automatic slack adjusters and will not be required to do anything to the braking system yourself. Even if your equipment does not come equipped with automatic slack adjusters, it is likely that your company will do all maintenance to the equipment and will not want their drivers touching anything.

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.


Answer

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.

This procedure will be required for your pre-trip inspection exam. It is a simple procedure, but doing it properly will be critically important to your CDL examiner. Know the proper air pressure where the knob on the dash should release and the warning light should come on. The exact numbers can vary from state to state so see your state's CDL manual for reference.

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.


Answer

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.

This procedure will be required for your pre-trip inspection exam. It is a simple procedure, but doing it properly will be critically important to your CDL examiner. Know the proper air pressure where the knob on the dash should release and the warning light should come on. The exact numbers can vary from state to state so see your state's CDL manual for reference.

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.


Answer

B. Less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles.

This knowledge will very likely be required for your pre-trip inspection exam and your written CDL test. It is possible that this number could vary from state to state, so check your state's CDL manual to be certain.

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

A. Yes
B. No


Answer

B. No

Pretty self-explanatory. I once had a truck roll through a parking lot and smash into the front of my truck because the driver was so tired that he forgot to set his parking brake before he went to sleep. It was a mess.

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


Answer

B. No

Again, pretty self-explanatory. Always set your brakes immediately when you park.

9. How often should you drain air tanks?

A. Daily
B. Weekly
C. Monthly


Answer

A. Daily

Most modern tractor trailers have automatic air-dryers on them which will release small amounts of air in set intervals so that it is rather unecessary to drain your tanks nearly as often as it used to be. But you will still likely find these questions asked on the pre-trip exam and on written tests.

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


Answer

A. True

Engine brakes, or "Jake Brakes" or "Jakes" as they are commonly referred to, are the greatest invention of all time when it comes to tractor trailers (well, maybe not of all time). When you are descending down a hill or a mountain, if you are in the proper gear it is almost always possible to have the Jake Brake hold you back entirely without the use of your brake pedal at all. It takes some experience and trial and error to figure out what gear to be in based on your weight and the grade of the hill, but if done properly can be very safe.

Be warned, though - you should not use your Jake Brakes in poor road conditions like wet or slippery roads. Also, you must be very careful in turns. Consult your CDL training manual and your instructors for the proper techniques for using Jake Brakes.

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.


Answer

B. Over heating, low air pressure, and not relying on the engine braking effect.

Excessive heat is the number one enemy of brakes. There are a number of factors that can lead to your brakes overheating, including improper braking techniques and improper brake adjustment. A brake that is adjusted too tightly will drag, creating a lot of excess heat. I have seen brakes catch on fire numerous times because of this. If some brakes are adjusted too loosely it can cause the other brakes on the truck to do the majority of the work, causing overheating in the properly adjusted brakes.

If the air pressure gets too low to certain brakes - possibly from a leak in the system - it can also cause your brakes to drag, creating excess heat.

Engine brakes, or "Jake Brakes" or "Jakes" as they are commonly referred to, are the greatest invention of all time when it comes to tractor trailers (well, maybe not of all time). When you are descending down a hill or a mountain, if you are in the proper gear it is almost always possible to have the Jake Brake hold you back entirely without the use of your brake pedal at all. It takes some experience and trial and error to figure out what gear to be in based on your weight and the grade of the hill, but if done properly can be very safe.

Be warned, though - you should not use your Jake Brakes in poor road conditions like wet or slippery roads. Also, you must be very careful in turns. Consult your CDL training manual and your instructors for the proper techniques for using Jake Brakes.

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.


Answer

A. So you only have to apply the brake just hard enough to feel a definite slowdown.

If you're in the proper gear before going down a hill and you're using your Jake Brakes, you will only have to use the brake pedal a small amount, if at all. You don't want to be shifting gears while going downhill if you can help it, and you don't want to use your brake pedal too much or you'll create too much heat in the braking system.

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.


Answer

B. The brakes could fail.

The biggest problem with oil and water in the braking system is ice. Ice can form in the lines causing a total blockage of air to the brake chambers. This can either prevent your parking brakes from releasing, or it can prevent the service air from getting to the brake chamber, causing your brakes to fail. It can also clog up the air release valve, causing your brakes to drag even after you've released the brake pedal.

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.


Answer

C. To reduce the risk of ice in the brake valves and other parts in cold weather.

An alcohol evaporator is a very nice thing to have.

The biggest problem with oil and water in the braking system is ice. Ice can form in the lines causing a total blockage of air to the brake chambers. This can either prevent your parking brakes from releasing, or it can prevent the service air from getting to the brake chamber, causing your brakes to fail. It can also clog up the air release valve, causing your brakes to drag even after you've released the brake pedal.

15. What is another name for controlled braking?

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


Answer

B. Squeeze braking

If you control your speed, descend hills in the proper gear, use your Jake Brakes properly, get the proper maintenance done to your tractor and trailers, and follow all braking procedures properly, you will rarely have any problems with your brake system or with getting down the mountains.

CDL Practice Tests

More CDL Training Resources From TruckingTruth

High Road Practice Questions

We've taken the questions from the High Road Training Program and put them in their own section of the site. The quizzes also contain quotes from the CDL manual and advice from TruckingTruth.

CDL Practice Test Android App

We've also converted the quizzes from the High Road Training Program into an Android app. Now you can practice taking the quizzes when you're on the go! The quizzes also contain quotes from the CDL manual and advice from TruckingTruth.

Tips On Passing The CDL Exam

Passing your CDL exam involves taking written exams, oral exams, and of course driving and backing tests. We have a series of articles that will help you understand what you're being tested for and how to give yourself the best chance of passing.

Join Us!

We have an awesome set of tools that will help you understand the trucking industry and prepare for a great start to your trucking career. Not only are they fantastic, but everything we offer here at TruckingTruth is 100% free - no strings attached! So sign up now and get instant access to our member's section, including:
High Road Training Program Logo
  • The High Road Training Program
  • The High Road Article Series
  • The Friendliest Trucker's Forum Ever!
  • Private messaging for more personal questions
  • Email Updates When New Articles Are Posted

About Us

Picture Of Brett Aquila
TruckingTruth was founded by Brett Aquila (that's me!), a 15 year truck driving veteran, in January 2007. After 15 years on the road I wanted to help people understand the trucking industry and everything that came with the career and lifestyle of an over the road trucker. We'll help you make the right choices and prepare you for a great start to your trucking career.