25 realistic Air Brakes practice questions for Nebraska CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 of 25 · Operation
When the brake pedal is pushed harder, what happens?
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A.
Brake pedal must be pumped to release
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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 2 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 3 of 25 · Compressor
Which of these systems removes water and oil from compressed air?
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A.
Air dryer
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B.
Brake chamber
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C.
Slack adjuster
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D.
S-cam
Correct answer: A.
The air dryer removes moisture and oil from the air before it reaches the storage tanks.
Question 4 of 25 · Parking
Why should you use chocks?
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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 5 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 6 of 25 · Gauges
The supply pressure gauge tells you:
-
A.
The pressure of brake fluid
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B.
How much air pressure is in the air tanks
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C.
The trailer brake pressure
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D.
The temperature of the brake
Correct answer: B.
The supply gauge shows the air pressure available in the storage tanks.
Question 7 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 8 of 25 · Parking
When parking a tractor-trailer with spring brakes:
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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 9 of 25 · Combination Air
What component prevents air pressure loss in case of a leak between the tractor and trailer?
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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 10 of 25 · Tanks
What is the safety valve set to open at?
-
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 11 of 25 · Operation
When making a normal stop, what should you do?
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A.
Push the brake pedal down so you can stop at a safe place and remain in control
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B.
Pump the brakes
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C.
Use only the parking brake
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D.
Use only the trailer hand valve
Correct answer: A.
Apply pressure progressively to come to a smooth, safe stop while maintaining control.
Question 12 of 25 · Gauges
You should know your vehicle is properly equipped with which gauge?
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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 13 of 25 · Emergency Maneuvers
Stab braking includes which of these steps?
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A.
Apply brakes fully, release when wheels lock, reapply when wheels begin rolling
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B.
Apply brakes lightly until they fade
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C.
Pump the brakes rapidly
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D.
Apply the parking brake repeatedly
Correct answer: A.
Stab braking: full application, release on lockup, reapply when wheels turn again.
Question 14 of 25 · Inspection
How much air pressure (psi) loss is allowed per minute for single vehicles with the engine off and brakes released?
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A.
Up to 1 psi
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B.
Up to 2 psi
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C.
Up to 3 psi
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D.
Up to 4 psi
Correct answer: B.
Single vehicles allow up to 2 psi per minute with brakes released; up to 3 psi with brakes applied.
Question 15 of 25 · Inspection
How much air pressure (psi) loss is allowed per minute for combination vehicles with the engine off and brakes released?
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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 16 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 17 of 25 · System Basics
Air brake systems combine three different braking systems. They are:
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A.
Service, parking, and emergency brakes
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B.
Hydraulic, parking, and emergency brakes
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C.
Front, rear, and trailer brakes
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D.
Disc, drum, and parking brakes
Correct answer: A.
The three are: the service brake, the parking brake, and the emergency brake.
Question 18 of 25 · System Basics
Air brakes use compressed air to:
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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 19 of 25 · Compressor
If the air compressor governor is not working properly:
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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 20 of 25 · Spring Brakes
Where are spring brakes typically NOT found?
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A.
Front steering axle
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B.
Drive axles
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C.
Trailer axles
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D.
Tractor rear axles
Correct answer: A.
Spring brakes are found on drive axles and trailer axles, not normally on the steering axle.
Question 21 of 25 · Inspection
When checking air brake adjustment, the engine should be:
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A.
Running at idle
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B.
Off with the brakes released
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C.
Running at high RPM
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D.
Off with the brakes applied
Correct answer: B.
Inspect with the engine off and the brakes released so chambers are not pressurized.
Question 22 of 25 · Foundation Brakes
When checking the slack adjuster on S-cam brakes, the slack adjuster should not move more than:
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A.
1/2 inch
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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 23 of 25 · ABS
What is the proper procedure for an air brake stop in a vehicle equipped with ABS?
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A.
Pump the brakes
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B.
Press the brake pedal firmly and steadily; let ABS work
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C.
Disengage ABS first
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D.
Apply the parking brake
Correct answer: B.
With ABS, brake firmly and steadily. ABS prevents wheel lockup automatically.
Question 24 of 25 · Inspection
How can you check the slack adjusters?
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A.
Pull hard on each slack adjuster — they should not move more than about 1 inch
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B.
Listen for air leaks at each chamber
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C.
Watch the brake light flash
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D.
Look at the brake pedal travel
Correct answer: A.
Pulling hard, slack adjusters should not move more than about 1 inch where the push rod attaches.
Question 25 of 25 · ABS
Which is true about ABS?
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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.
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 Nebraska Department of Motor Vehicles administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Nebraska CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Inspection: 5 questions
- Parking: 3 questions
- Operation: 2 questions
- Compressor: 2 questions
- Gauges: 2 questions
- Combination Air: 2 questions
- System Basics: 2 questions
- ABS: 2 questions
- Speed Management: 1 question
- Tanks: 1 question
- Emergency Maneuvers: 1 question
- Spring Brakes: 1 question
- Foundation Brakes: 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 Nebraska 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.