Kentucky Air Brakes CDL Practice Test
25 realistic Air Brakes practice questions for Kentucky CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 of 25 · Spring Brakes
If your vehicle has dual parking control valves, you can use pressure from a separate tank to:
-
A.
Charge the trailer brakes
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B.
Release the spring brakes to move a short distance if the main system loses air
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C.
Operate the steering
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D.
Power the air horn
Correct answer: B.
Dual parking control valves let you temporarily release spring brakes from a backup tank.
Question 2 of 25 · Combination Air
What does the trailer hand valve do?
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A.
Operates only the trailer brakes
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B.
Operates both the tractor and trailer brakes
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C.
Engages the parking brakes
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D.
Lowers the landing gear
Correct answer: A.
The hand valve (trolley valve) controls only the trailer service brakes.
Question 3 of 25 · Inspection
When should you push the brake pedal during the air brake test?
-
A.
Only when the engine is running
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B.
When checking the compressor
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C.
When the engine is off, to check that pressure does not drop too quickly
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D.
During the brake light test only
Correct answer: C.
Apply firm pressure with the engine off to check for excessive air leakage from the system.
Question 4 of 25 · Combination Air
What component prevents air pressure loss in case of a leak between the tractor and trailer?
-
A.
Tractor protection valve
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B.
Air dryer
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C.
Foot brake valve
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D.
Modulating valve
Correct answer: A.
The tractor protection valve closes off the trailer air supply during a leak, protecting tractor air.
Question 5 of 25 · Speed Management
Air brake lag is:
-
A.
The time required for the brakes to apply after the pedal is pressed
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B.
The amount of free play in the brake pedal
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C.
The distance traveled before the brakes release
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D.
The time to drain air tanks
Correct answer: A.
Lag is the time between pressing the pedal and the brakes actually applying — about 0.4 seconds.
Question 6 of 25 · Operation
When the brake pedal is pushed harder, what happens?
-
A.
Brake pedal must be pumped to release
-
B.
More air goes to the brakes; less foot pressure releases air
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C.
It locks the brakes permanently
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D.
The brakes do not respond
Correct answer: B.
The harder you push, the more air pressure is sent to the brake chambers.
Question 7 of 25 · ABS
If the ABS warning light comes on, you should:
-
A.
Stop driving immediately
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B.
Continue, knowing brakes still work — but ABS may not
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C.
Pump the brakes constantly
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D.
Engage the trailer hand valve
Correct answer: B.
You still have normal braking; only the ABS function may be off. Get it fixed soon.
Question 8 of 25 · ABS
Which is correct about ABS-equipped vehicles?
-
A.
You should never use ABS
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B.
ABS works only at high speeds
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C.
You should drive the same way as without ABS, except brake firmly without pumping in emergencies
-
D.
ABS makes stopping distance shorter
Correct answer: C.
Drive normally; in emergencies, brake firmly and let ABS prevent lockup. ABS doesn't shorten stopping distance.
Question 9 of 25 · Tanks
Why must air tanks be drained?
-
A.
To remove fuel from the system
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B.
To remove water and compressor oil that can damage the brakes
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C.
To inspect the tanks
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D.
To increase pressure
Correct answer: B.
Water and oil collect in tanks. They damage seals and can freeze in cold weather.
Question 10 of 25 · Spring Brakes
What does the term "modulating control valve" refer to?
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A.
A valve to regulate engine RPM
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B.
A hand-operated valve that gradually applies the spring brakes
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C.
A device that drains air tanks
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D.
The trailer hand valve
Correct answer: B.
A modulating control valve allows gradual application of spring brakes for emergency or backup use.
Question 11 of 25 · Parking
You should never use the trailer hand valve to park because:
-
A.
It is not strong enough
-
B.
The air could leak out, releasing the brakes
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C.
It only applies the front brakes
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D.
It is not connected to the trailer
Correct answer: B.
If air leaks out, the brakes release. Always use the parking brake to park.
Question 12 of 25 · Parking
Why should you use chocks?
-
A.
When parking on a hill
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B.
On level ground when leaving the vehicle
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C.
Whenever brakes might fail or it's required by company policy or law
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D.
All of the above
Correct answer: D.
Chocks should be used when required to prevent vehicle movement.
Question 13 of 25 · Tanks
Air brake equipped vehicles must have:
-
A.
Only an emergency brake
-
B.
A drain valve in each air tank
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C.
Hydraulic backup
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D.
Disc brakes only
Correct answer: B.
Each air tank has a drain valve to remove water and oil that collect in the tank.
Question 14 of 25 · Spring Brakes
Spring brakes will come on automatically when air pressure drops to:
-
A.
100 psi
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B.
60 psi or less
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C.
20 to 45 psi
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D.
10 psi
Correct answer: C.
Spring brakes typically apply automatically between 20 and 45 psi.
Question 15 of 25 · Compressor
If the air compressor governor is not working properly:
-
A.
Air compressor may not maintain enough pressure to safely operate the brakes
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B.
The brakes will work harder
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C.
Brakes may not release at all
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D.
There is no impact on safety
Correct answer: A.
A faulty governor can cause low system pressure or over-pressurization, both unsafe.
Question 16 of 25 · Tanks
What is the safety valve set to open at?
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A.
100 psi
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B.
125 psi
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C.
150 psi
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D.
180 psi
Correct answer: C.
The safety valve is normally set to open at 150 psi to protect the system from over-pressurization.
Question 17 of 25 · Foundation Brakes
When checking the slack adjuster on S-cam brakes, the slack adjuster should not move more than:
-
A.
1/2 inch
-
B.
1 inch
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C.
About 1 inch where the push rod attaches to it
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D.
2 inches
Correct answer: C.
When pulled by hand, the slack adjuster shouldn't move more than about an inch where the push rod attaches.
Question 18 of 25 · ABS
Which is true about ABS?
-
A.
It activates only on dry roads
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B.
It activates only when wheels are about to lock during heavy braking
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C.
It is the same as anti-skid
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D.
It eliminates the need for skid recovery
Correct answer: B.
ABS activates only when wheels are about to lock; otherwise the brake system operates normally.
Question 19 of 25 · Speed Management
Which of these statements is true about brake fading?
-
A.
Resting brakes does not bring back braking power
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B.
Brake fade is caused by drums getting too hot
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C.
Brake fade only affects new vehicles
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D.
Brake fade only happens with disc brakes
Correct answer: B.
Brake fade results from overheated drums or discs reducing friction and braking ability.
Question 20 of 25 · Foundation Brakes
Brake drums (or discs) must not have cracks longer than:
-
A.
1/4 the width of the friction area
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B.
1/2 the width of the friction area
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C.
3/4 the width of the friction area
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D.
Anything is acceptable
Correct answer: B.
Cracks longer than one-half the width of the friction area are a safety violation.
Question 21 of 25 · Compressor
Brake-system air pressure should build from 85 to 100 psi within:
-
A.
15 seconds in dual systems
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B.
30 seconds in dual systems
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C.
45 seconds in dual systems
-
D.
60 seconds in dual systems
Correct answer: C.
Build-up time should be within 45 seconds in dual air systems.
Question 22 of 25 · Spring Brakes
What does a "spring brake" mean?
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A.
A brake that operates only in spring weather
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B.
A mechanical brake that uses powerful springs to apply the brake when air pressure is lost
-
C.
A specific kind of disc brake
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D.
A backup hydraulic brake
Correct answer: B.
Spring brakes use powerful coil springs to apply the brakes when air pressure is lost.
Question 23 of 25 · Combination Air
When should the trailer brake hand valve be used?
-
A.
To park the trailer
-
B.
To test the trailer brakes only
-
C.
To stop the entire combination
-
D.
Never to keep the vehicle from rolling
Correct answer: B.
Use it only to test trailer brakes. Never use it to park or as a service brake during normal stops.
Question 24 of 25 · ABS
What is the proper procedure for an air brake stop in a vehicle equipped with ABS?
-
A.
Pump the brakes
-
B.
Press the brake pedal firmly and steadily; let ABS work
-
C.
Disengage ABS first
-
D.
Apply the parking brake
Correct answer: B.
With ABS, brake firmly and steadily. ABS prevents wheel lockup automatically.
Question 25 of 25 · Spring Brakes
Modulating control valves do what?
-
A.
Drain water from tanks
-
B.
Apply spring brakes gradually using a hand control
-
C.
Open the trailer supply line
-
D.
Switch to emergency brakes
Correct answer: B.
Modulating valves let you apply the spring brakes gradually rather than all at once.
About the Air Brakes Test
The Air Brakes test covers air brake system parts (compressor, governor, reservoirs, drain valves, alcohol evaporator, safety valve, brake pedal, foundation brakes), dual air brake systems, inspecting air brakes, using air brakes (normal stops, emergency stops, parking brakes), and proper procedures for braking on downgrades. If you fail or skip the Air Brakes test, your CDL is restricted to vehicles without full air brake systems.
The Kentucky Division of Driver Licensing administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Kentucky CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Spring Brakes: 5 questions
- ABS: 4 questions
- Combination Air: 3 questions
- Tanks: 3 questions
- Speed Management: 2 questions
- Parking: 2 questions
- Compressor: 2 questions
- Foundation Brakes: 2 questions
- Inspection: 1 question
- Operation: 1 question
How to use this practice test
- Read each question carefully. CDL questions are written precisely — small wording differences matter.
- Click "Show Answers & Explanations" only after answering every question. Don't peek mid-test.
- Read the explanation for every question, even ones you got right. The reasoning is more important than the answer.
- Repeat the test until you score 100%. The questions are deterministic per state, so you can track your improvement over multiple sessions.
- Move on to the other endorsement tests for Kentucky until you're ready for the official exam.
Tips specific to the Air Brakes test
Air Brakes questions test your ability to identify components, follow inspection sequences, and respond to system failures. Memorize the cut-in and cut-out pressures, the low-pressure warning thresholds, the pressure ranges at which spring brakes apply, and the maximum allowable air-loss rates. Many questions have nearly-identical wording, so precision matters.
Other practice tests for Kentucky