Illinois CDL Air Brakes Practice Test

The Illinois CDL Air Brakes test is a multiple-choice knowledge test you take for your commercial learner's permit. Illinois Secretary of State, Driver Services Department says you need 80% to pass (source).

Last updated Oct 15, 2026. FirstTry CDL is a CDL permit test prep site by Hive Asset Group, LLC.

By Scott Wilder, Editor

Last reviewed against the Illinois CDL manual on October 15, 2026

Scott Wilder on X | scottwilderhq.com

FirstTry CDL is a CDL permit test prep site by Hive Asset Group, LLC. It offers free exam-style practice tests built from the official CDL manual. FirstTry CDL is not affiliated with any DMV, FMCSA or government agency. Practice questions are exam-style questions built from the official CDL manual, not actual test questions.

Illinois CDL permit facts

What	What Illinois says	Source
Air Brakes pass mark	80%	Official page
Air Brakes test questions	25	Official page
CLP fee (adding a CLP to an Illinois non-CDL license, or renewing an original CLP)	$50	Official page
CLP fee (current CDL holder upgrading class or adding an endorsement)	$5	Official page
CLP validity	One year. An original CLP expires one year from the date it is first issued.	Official page
Test languages	CDL knowledge tests are given in English only. You can ask for the audio format, where the computer reads the test. After trying audio, you can ask facility management about an oral test read by staff.	Official page

20 question Air Brakes practice test

1. For the service brake test, you release the parking brake and roll forward at about what speed?
1. A. About 15 mph
2. B. About five mph
3. C. About 25 mph
4. D. About 45 mph
Show answer
Answer: B. About five mph
The test is done by moving forward slowly, about five mph, and applying the brakes firmly.
From the official CDL manual, section 5.3.3.
2. A heavily loaded vehicle's spring brakes come on after an air loss. Why does it take a long distance to stop?
1. A. The spring brakes do not work on all axles
2. B. The air tanks must refill before the brakes can work
3. C. The compressor stops pumping at that point
4. D. The service brakes lock the wheels at once
Show answer
Answer: A. The spring brakes do not work on all axles
Spring brakes do not work on all axles, so a heavily loaded vehicle takes a long distance to stop.
From the official CDL manual, section 5.4.7.
3. How does the stab braking method work?
1. A. Brake lightly and add more pressure slowly
2. B. Brake as hard as you can without locking the wheels
3. C. Hold the pedal down fully until the vehicle stops
4. D. Brake fully, release when wheels lock, brake fully again once they roll
Show answer
Answer: D. Brake fully, release when wheels lock, brake fully again once they roll
In stab braking you apply the brakes all the way, release when the wheels lock, and apply fully again as soon as the wheels roll.
From the official CDL manual, section 5.4.3.
4. Your truck has larger than minimum air tanks. How does that affect the pressure buildup time?
1. A. Buildup must be shorter than for normal tanks
2. B. Buildup can take longer and still be safe
3. C. Buildup must still be exactly 45 seconds or you fail
4. D. Tank size has no effect on buildup time
Show answer
Answer: B. Buildup can take longer and still be safe
Larger tanks take longer to fill, so a longer buildup time can still be safe. Check the manufacturer's specifications.
From the official CDL manual, section 5.3.3.
5. One brake on your truck is out of adjustment. What can happen to the other brakes?
1. A. They will cool down
2. B. They will work less
3. C. They will lock up for good
4. D. They can overheat and fade
Show answer
Answer: D. They can overheat and fade
The other brakes must do more work, so they can overheat and fade.
From the official CDL manual, section 5.4.5.
6. In Step 7 of the inspection, what do you do on an air brake vehicle?
1. A. Skip the brake check
2. B. Do the air brake checks instead of the hydraulic brake check
3. C. Do the hydraulic brake check only
4. D. Do both the air and hydraulic checks
Show answer
Answer: B. Do the air brake checks instead of the hydraulic brake check
The manual says to do these checks instead of the hydraulic brake check.
From the official CDL manual, section 5.3.3.
7. On older vehicles, the low air warning may come on before pressure falls below what?
1. A. The pressure at which the governor cuts in
2. B. A fixed 20 psi on every vehicle
3. C. One half of the governor cutout pressure
4. D. The pressure at which the safety valve opens
Show answer
Answer: C. One half of the governor cutout pressure
For older vehicles the manual gives one half of the compressor governor cutout pressure.
From the official CDL manual, section 5.1.11.
8. Which air brake system applies and releases the brakes when you use the brake pedal in normal driving?
1. A. The service brake system
2. B. The parking brake system
3. C. The emergency brake system
4. D. The foundation brake system
Show answer
Answer: A. The service brake system
The service brakes work when you press and release the brake pedal.
From the official CDL manual, section 5.
9. Only your tractor has ABS. During a stop, the trailer begins to swing out. What should you do?
1. A. Press harder on the brake pedal
2. B. Let up on the brakes if you can safely do so
3. C. Turn the steering wheel sharply to the side
4. D. Apply the parking brake to hold the trailer in line
Show answer
Answer: B. Let up on the brakes if you can safely do so
With ABS on the tractor only, watch the trailer and let up on the brakes if it begins to swing out and it is safe to do so.
From the official CDL manual, section 5.4.2.
10. A truck was built in 1996. What kind of slack adjusters does it have?
1. A. Manual slack adjusters only
2. B. No slack adjusters
3. C. Automatic slack adjusters
4. D. Slack adjusters on the front axle only
Show answer
Answer: C. Automatic slack adjusters
The manual says all vehicles built since 1994 have automatic slack adjusters.
From the official CDL manual, section 5.3.2.
11. When you firmly apply the service brakes during your test, what should you watch for?
1. A. Only that the brake lights come on
2. B. Only that the vehicle stops in less than a second
3. C. Pulling to one side, an unusual feel, or delayed stopping
4. D. Only that the air gauge reads 125 psi
Show answer
Answer: C. Pulling to one side, an unusual feel, or delayed stopping
The test shows problems such as pulling to one side, odd pedal feel, or delayed stopping.
From the official CDL manual, section 5.3.3.
12. How long can a crack in a brake drum be before the drum fails the inspection?
1. A. Any crack that does not reach the edge of the drum
2. B. No longer than one half the width of the friction area
3. C. No longer than the full width of the friction area
4. D. No longer than one inch
Show answer
Answer: B. No longer than one half the width of the friction area
Drums and discs must not have cracks longer than one half the width of the friction area.
From the official CDL manual, section 5.3.2.
13. What does an alcohol evaporator do?
1. A. Reduces the risk of ice in brake valves and parts
2. B. Removes the oil from the compressed air
3. C. Raises the air pressure in the tanks
4. D. Cools the compressor while it runs
Show answer
Answer: A. Reduces the risk of ice in brake valves and parts
It puts alcohol into the air system to help prevent ice in cold weather.
From the official CDL manual, section 5.1.5.
14. How does an air brake system turn on the brake lights?
1. A. A cable tied to the brake pedal linkage
2. B. A switch built into the governor
3. C. An electric switch that works by air pressure
4. D. The low air pressure warning switch
Show answer
Answer: C. An electric switch that works by air pressure
The stop light switch is electric but is operated by air pressure.
From the official CDL manual, section 5.1.12.
15. In controlled braking, you released the brakes to make a larger steering change. When do you re-apply them?
1. A. After you stop steering for ten seconds
2. B. As soon as you can
3. C. Only after the vehicle stops
4. D. Never, coast to a stop
Show answer
Answer: B. As soon as you can
Re-apply the brakes as soon as you can.
From the official CDL manual, section 5.4.3.
16. What is the controlled braking method?
1. A. Pressing the pedal all the way, then releasing when the wheels lock
2. B. Tapping the pedal lightly many times in a row
3. C. Braking as hard as you can without locking the wheels
4. D. Using only the engine brake and the parking brake
Show answer
Answer: C. Braking as hard as you can without locking the wheels
In controlled braking you apply the brakes as hard as you can without locking the wheels.
From the official CDL manual, section 5.4.3.
17. When is controlled braking possible with air brakes?
1. A. Only after the spring brakes apply
2. B. Only on dry roads
3. C. Only while enough air remains in the tanks
4. D. At any air pressure
Show answer
Answer: C. Only while enough air remains in the tanks
Controlled braking works only while enough air is in the tanks.
From the official CDL manual, section 5.4.7.
18. When may a driver fully apply the brakes in an emergency stop with ABS?
1. A. Whenever only the tractor has ABS
2. B. Whenever only the trailer has ABS
3. C. Whenever the road is dry
4. D. When driving a straight truck or combination with working ABS on all axles
Show answer
Answer: D. When driving a straight truck or combination with working ABS on all axles
The one exception is a driver who always drives vehicles with working ABS on all axles, who can fully apply the brakes in an emergency stop.
From the official CDL manual, section 5.4.2.
19. How may an air compressor be cooled?
1. A. By air or by the engine cooling system
2. B. By the brake fluid only
3. C. By the air tanks only
4. D. By the governor
Show answer
Answer: A. By air or by the engine cooling system
The compressor may be air cooled or cooled by the engine cooling system.
From the official CDL manual, section 5.1.1.
20. Why must emergency and parking brakes be held on by mechanical force?
1. A. Air is too expensive
2. B. Springs are lighter than air lines
3. C. Air pressure can eventually leak away
4. D. Drivers forget to use air
Show answer
Answer: C. Air pressure can eventually leak away
Air pressure can eventually leak away, so these brakes must be held on mechanically.
From the official CDL manual, section 5.1.14.

Illinois CDL Air Brakes questions

What score do you need to pass the Illinois CDL Air Brakes test?

Illinois says you need 80% to pass the Air Brakes test.

Is the Illinois CDL Air Brakes test the same as other states?

The Air Brakes test is built on the same federal knowledge standards in every state, so the topics match the CDL manual. Illinois uses its own handbook: https://www.ilsos.gov/content/dam/publications/pdf_publications/dsd_cdl10.pdf

How much does a CDL permit cost in Illinois?

CLP fee (adding a CLP to an Illinois non-CDL license, or renewing an original CLP): $50.

What languages is the Illinois CDL knowledge test offered in?

CDL knowledge tests are given in English only. You can ask for the audio format, where the computer reads the test. After trying audio, you can ask facility management about an oral test read by staff.

Sources

https://www.ilsos.gov/content/dam/publications/pdf_publications/dsd_cdl10.pdf
https://www.ilsos.gov/departments/drivers/basicfees.html
https://www.ilsos.gov/departments/drivers/drivers-license/cdl/cdl.html
Manual sections used in this test: 5; 5.1.1; 5.1.11; 5.1.12; 5.1.14; 5.1.5; 5.3.2; 5.3.3; 5.4.2; 5.4.3; 5.4.5; 5.4.7

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