25 realistic Air Brakes practice questions for Alaska CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 of 25 · Tanks
Air brake equipped vehicles must have:
-
A.
Only an emergency brake
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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 2 of 25 · Inspection
Air leaks during a static brake check could indicate:
-
A.
Faulty brake chambers or air lines
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B.
Engine problems
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C.
Bad tires
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D.
Low fuel pressure
Correct answer: A.
Air leaks usually indicate faulty chambers, lines, fittings, or valves.
Question 3 of 25 · Inspection
How much air pressure (psi) loss is allowed per minute for combination vehicles with the engine off and brakes released?
-
A.
Up to 1 psi
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B.
Up to 2 psi
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C.
Up to 3 psi
-
D.
Up to 4 psi
Correct answer: B.
Combination vehicles allow up to 3 psi loss per minute with brakes released; with brakes applied, up to 4 psi.
Question 4 of 25 · System Basics
Air brakes use compressed air to:
-
A.
Lubricate the wheels
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B.
Make the brakes work
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C.
Cool the engine
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D.
Increase fuel pressure
Correct answer: B.
Compressed air is the working fluid that applies the brakes in an air brake system.
Question 5 of 25 · Spring Brakes
What happens if the air pressure drops to between 20 and 45 psi?
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A.
Nothing happens
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B.
The compressor cuts in
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C.
Spring brakes apply automatically
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D.
The trailer is released
Correct answer: C.
Spring brakes typically apply automatically in this range, stopping the vehicle.
Question 6 of 25 · Spring Brakes
Spring brakes:
-
A.
Are released by air pressure
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B.
Are applied by air pressure
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C.
Should be tested by pumping the brake pedal
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D.
Are only on the trailer
Correct answer: A.
Spring brakes are held off (released) by air pressure. When pressure drops, the springs apply the brakes.
Question 7 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
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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 8 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 9 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 10 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 11 of 25 · ABS
Which is correct about ABS-equipped vehicles?
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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
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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 12 of 25 · Combination Air
When should the trailer brake hand valve be used?
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A.
To park the trailer
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B.
To test the trailer brakes only
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C.
To stop the entire combination
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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 13 of 25 · Emergency Maneuvers
Which of these statements is true about emergency braking?
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A.
Pumping the brakes is always best
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B.
Stab braking is for non-ABS vehicles in emergencies
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C.
You should brake harder than necessary
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D.
Stab braking is the same as pumping
Correct answer: B.
Stab braking applies to non-ABS systems: full application until lockup, then release.
Question 14 of 25 · Speed Management
Why is it important not to over-apply the brakes?
-
A.
Brakes can overheat and fade
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B.
Brakes will lock and skid
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C.
You may lose control of the vehicle
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D.
All of the above
Correct answer: D.
Over-application leads to fade, lockup, skidding, and loss of control.
Question 15 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 16 of 25 · Inspection
When should you push the brake pedal during the air brake test?
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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 17 of 25 · Speed Management
Air brake lag is:
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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 18 of 25 · Parking
When parking a tractor-trailer with spring brakes:
-
A.
Use the trailer hand valve only
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B.
Use the parking brake only — do not push the brake pedal down to hold the vehicle
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C.
Press the foot brake and then engage the parking brake
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D.
Set both brakes simultaneously
Correct answer: B.
Don't push the pedal down because it will cause damage. Set the parking brakes only.
Question 19 of 25 · Compressor
What is the purpose of an alcohol evaporator?
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A.
Reduce alcohol in the driver's system
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B.
Reduce risk of frozen brake valves and lines in cold weather
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C.
Improve fuel economy
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D.
Lubricate the air compressor
Correct answer: B.
Alcohol injected into the air system reduces freeze-up risk in cold conditions.
Question 20 of 25 · Parking
On a tractor with no trailer attached, the tractor parking brake control:
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A.
Will lock and unlock the front brakes
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B.
Will set or release the spring brakes on the tractor
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C.
Has no function
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D.
Operates only the trailer brakes
Correct answer: B.
The yellow diamond control sets and releases the tractor parking (spring) brakes.
Question 21 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 22 of 25 · Inspection
Which of the following must be checked before operating an air-braked vehicle?
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A.
Low air pressure warning signal
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B.
Spring brakes set automatically
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C.
Air leakage rate
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D.
All of the above
Correct answer: D.
All of these are required pre-trip air brake checks.
Question 23 of 25 · Parking
You should never use the trailer hand valve to park because:
-
A.
It is not strong enough
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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 24 of 25 · Gauges
You should know your vehicle is properly equipped with which gauge?
-
A.
Two air pressure gauges (or one with two needles) showing pressure in primary and secondary tanks
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B.
A vacuum gauge
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C.
A voltage meter only
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D.
A coolant gauge only
Correct answer: A.
Dual air systems require gauges for both primary and secondary air supplies.
Question 25 of 25 · System Basics
Which of these is NOT a part of the basic air brake system?
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A.
Air compressor
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B.
Air storage tanks
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C.
Hydraulic master cylinder
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D.
Foot brake valve
Correct answer: C.
Hydraulic master cylinders belong to hydraulic brake systems, not air brake systems.
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 Alaska Division of Motor Vehicles administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Alaska CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Spring Brakes: 6 questions
- Inspection: 4 questions
- Parking: 3 questions
- Tanks: 2 questions
- System Basics: 2 questions
- ABS: 2 questions
- Speed Management: 2 questions
- Combination Air: 1 question
- Emergency Maneuvers: 1 question
- Compressor: 1 question
- Gauges: 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 Alaska 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.