Missouri CDL Air Brakes Practice Test

The Missouri CDL Air Brakes test is a multiple-choice knowledge test you take for your commercial learner's permit. Missouri Department of Revenue (licenses) and Missouri State Highway Patrol (testing) says you need 80% to pass (source).

Last updated Oct 7, 2026. FirstTry CDL is a CDL permit test prep site by Hive Asset Group, LLC.

By Scott Wilder, Editor

Last reviewed against the Missouri CDL manual on October 7, 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.

Missouri CDL permit facts

What	What Missouri says	Source
Air Brakes pass mark	80%	Official page
Air Brakes test questions	25	Official page
CLP fee	$44, which is a $25 written test fee, a $10 permit fee and a $9 office fee. Pay by card, check or cash	Official page
Duplicate CLP fee	$14	Official page
CLP validity	360 days. It cannot be renewed. When it ends you must pass all the knowledge tests again for a new CLP	Official page
CLP holding period	Hold the CLP at least 14 days before the CDL skills test. The test paper alone is not a permit	Official page
Knowledge test retest rule	You may retake a failed test right away, but the examiner can make you come back on a later day	Official page
Test languages	Commercial knowledge tests are English only, and no translators are allowed (RSMo 302.720).	Official page

20 question Air Brakes practice test

1. 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.
2. It starts to rain. Where should you set the front brake limiting valve control?
1. A. Normal
2. B. Slippery
3. C. Halfway between the two settings
4. D. Slippery, but only if there is ice
Show answer
Answer: A. Normal
The control should stay in normal for normal stopping power.
From the official CDL manual, section 5.1.13.
3. You start the low pressure warning test. When should you shut the engine off?
1. A. When the warning signal is already on
2. B. When the tank pressure is at zero
3. C. Right after the spring brakes come on
4. D. When there is enough pressure that the warning signal is off
Show answer
Answer: D. When there is enough pressure that the warning signal is off
Shut the engine off when pressure is high enough that the warning signal is not on.
From the official CDL manual, section 5.3.3.
4. The low air warning has come on during your check. How do you continue to test the spring brakes?
1. A. Restart the engine and build pressure to 125 psi
2. B. Push the parking brake knob in and drive forward
3. C. Keep stepping on and off the brake pedal to lower pressure more
4. D. Open the drain valves on the wet tank
Show answer
Answer: C. Keep stepping on and off the brake pedal to lower pressure more
You keep fanning the brake pedal to reduce tank pressure until the spring brakes come on.
From the official CDL manual, section 5.3.3.
5. Your low air pressure warning comes on. Why is it safer to stop right away?
1. A. The spring brakes will give you smoother stops
2. B. The compressor works better at a standstill
3. C. You can still use the foot brakes while enough air remains
4. D. The warning goes off once you slow down
Show answer
Answer: C. You can still use the foot brakes while enough air remains
It is much safer to stop while there is enough air in the tanks to use the foot brakes.
From the official CDL manual, section 5.4.7.
6. What does ABS do?
1. A. It makes the compressor pump faster
2. B. It keeps your wheels from locking up during hard braking
3. C. It drains the air tanks
4. D. It sets the parking brake
Show answer
Answer: B. It keeps your wheels from locking up during hard braking
ABS is a computerized system that keeps wheels from locking up during hard brake applications.
From the official CDL manual, section 5.1.16.
7. Engine off, parking brake released, system fully charged. What is the air loss limit per minute for a combination vehicle?
1. A. One psi
2. B. Three psi
3. C. Two psi
4. D. Five psi
Show answer
Answer: B. Three psi
With the engine off and the parking brake released, a combination vehicle should lose less than three psi in one minute.
From the official CDL manual, section 5.3.3.
8. Your air compressor is belt-driven. What do you check on the belt during the engine compartment check?
1. A. Only that it is the same color as the other belts
2. B. Its length compared with a chart in the cab
3. C. Its condition and how tight it is
4. D. Nothing, because the governor protects the compressor
Show answer
Answer: C. Its condition and how tight it is
The manual says to check the condition and tightness of a belt-driven compressor's belt.
From the official CDL manual, section 5.3.1.
9. Your ABS stops working while you drive. What do you have, and what should you do?
1. A. Regular brakes; drive normally and get the system serviced soon
2. B. No brakes at all; stop at once and call for a tow
3. C. Only rear brakes; drive very slowly
4. D. Weaker brakes; pump the pedal on every stop
Show answer
Answer: A. Regular brakes; drive normally and get the system serviced soon
The regular brakes still work, so you drive normally and have the ABS serviced soon.
From the official CDL manual, section 5.4.2.
10. 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.
11. Why does an air brake system have a stop light switch?
1. A. To warn drivers behind you when you brake
2. B. To warn you of low air pressure
3. C. To release the spring brakes
4. D. To drain the air tanks
Show answer
Answer: A. To warn drivers behind you when you brake
Drivers behind you must be warned when you put your brakes on, so the switch turns on the brake lights.
From the official CDL manual, section 5.1.12.
12. Which pair shows the typical pressures where the compressor starts and stops pumping?
1. A. Starts at about 100 psi, stops at about 125 psi
2. B. Starts at about 85 psi, stops at about 100 psi
3. C. Starts at about 125 psi, stops at about 100 psi
4. D. Starts at about 20 psi, stops at about 45 psi
Show answer
Answer: A. Starts at about 100 psi, stops at about 125 psi
The manual says pumping should start at about 100 psi and stop at about 125 psi (check manufacturer's specifications).
From the official CDL manual, section 5.3.3.
13. What two things in compressed air are bad for an air brake system?
1. A. Dust and sand
2. B. Alcohol and antifreeze
3. C. Water and compressor oil
4. D. Fuel and grease
Show answer
Answer: C. Water and compressor oil
Compressed air usually has some water and compressor oil in it, which is bad for the system.
From the official CDL manual, section 5.1.4.
14. Air brakes are really three braking systems in one. Which choice names all three?
1. A. Service, trailer, and parking
2. B. Primary, secondary, and parking
3. C. Foundation, service, and emergency
4. D. Service, parking, and emergency
Show answer
Answer: D. Service, parking, and emergency
The manual lists the service brake, the parking brake, and the emergency brake systems.
From the official CDL manual, section 5.
15. In a dual air system, the low pressure warning must come on before which tank's pressure drops too low?
1. A. Only the primary tank
2. B. The tank with the highest pressure
3. C. The tank with the lowest pressure
4. D. Only the tank on the trailer
Show answer
Answer: C. The tank with the lowest pressure
In dual systems, the 55 psi limit applies to the tank with the lowest air pressure.
From the official CDL manual, section 5.3.3.
16. At about what tank pressure does the governor stop the compressor from pumping air?
1. A. 125 psi
2. B. 100 psi
3. C. 150 psi
4. D. 55 psi
Show answer
Answer: A. 125 psi
The cut-out level is around 125 psi.
From the official CDL manual, section 5.1.2.
17. A truck has ABS on only one axle. What does the manual say about that?
1. A. It is not allowed
2. B. It still gives you more control while braking
3. C. It gives no benefit
4. D. It makes the brakes grab
Show answer
Answer: B. It still gives you more control while braking
The manual says even one ABS axle gives more control during braking.
From the official CDL manual, section 5.4.2.
18. An older trailer may have ABS but it is hard to tell. What can you look for under the vehicle?
1. A. The electronic control unit and wheel speed sensor wires
2. B. A second air compressor
3. C. An alcohol evaporator
4. D. A front brake limiting valve
Show answer
Answer: A. The electronic control unit and wheel speed sensor wires
Look for the electronic control unit (ECU) and wheel speed sensor wires from the back of the brakes.
From the official CDL manual, section 5.1.16.
19. An automatic front wheel limiting valve cuts air to the front brakes unless application pressure is at least what?
1. A. 20 psi
2. B. 100 psi
3. C. 55 psi
4. D. 125 psi
Show answer
Answer: C. 55 psi
Automatic valves reduce front brake air except at 55 psi or more application pressure.
From the official CDL manual, section 5.1.13.
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.

Missouri CDL Air Brakes questions

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

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

Is the Missouri 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. Missouri uses its own handbook: https://dor.mo.gov/forms/Commerical_Driver_License.pdf

How much does a CDL permit cost in Missouri?

CLP fee: $44, which is a $25 written test fee, a $10 permit fee and a $9 office fee. Pay by card, check or cash.

What languages is the Missouri CDL knowledge test offered in?

Commercial knowledge tests are English only, and no translators are allowed (RSMo 302.720).

Sources

https://dor.mo.gov/resources/official-final-rules/documents/12_CSR_10-24_300_CDL_Written_Examinations_Final.pdf
https://dor.mo.gov/driver-license/issuance/commercial/
https://dor.mo.gov/driver-license/documents/feechart.pdf
https://dor.mo.gov/driver-license/issuance/commercial/cdl-permit.html
https://dor.mo.gov/faq/driver-license/cdl-clp.html
https://dor.mo.gov/forms/Commerical_Driver_License.pdf
Manual sections used in this test: 5; 5.1.12; 5.1.13; 5.1.14; 5.1.16; 5.1.2; 5.1.4; 5.3.1; 5.3.2; 5.3.3; 5.4.2; 5.4.7

Air Brakes practice test in nearby states