25 realistic Air Brakes practice questions for New Hampshire CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 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
-
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 2 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 3 of 25 · Combination Air
When should the trailer brake hand valve be used?
-
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 4 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
-
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 5 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 6 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 7 of 25 · Inspection
When checking air brake adjustment, the engine should be:
-
A.
Running at idle
-
B.
Off with the brakes released
-
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 8 of 25 · Inspection
When should you check the parking brake?
-
A.
Whenever you start a trip
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B.
Once a month
-
C.
Only at inspections
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D.
When the brake warning light is on
Correct answer: A.
Test the parking brake every trip — apply the brake and try to move forward gently.
Question 9 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 10 of 25 · Bobtail
What is bobtail braking?
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A.
Braking with no trailer attached, when the tractor brakes harder than usual
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B.
A type of trailer-only braking
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C.
Emergency braking on doubles
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D.
A way to test the parking brake
Correct answer: A.
A bobtail tractor (without a trailer) brakes hard because it has less weight on the drive axles.
Question 11 of 25 · Compressor
The air compressor governor controls:
-
A.
When the air compressor will pump air into the storage tanks
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B.
When the brakes are applied
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C.
When emergency brakes engage
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D.
How fast the air compressor turns
Correct answer: A.
The governor cuts the compressor in and out to keep the system within its operating pressure range.
Question 12 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
-
D.
S-cam
Correct answer: A.
The air dryer removes moisture and oil from the air before it reaches the storage tanks.
Question 13 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 14 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
-
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 15 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
-
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 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 · Foundation Brakes
What's the most common type of foundation brake found on heavy vehicles?
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A.
Disc brakes
-
B.
Wedge drum brakes
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C.
S-cam drum brakes
-
D.
Air-over-hydraulic
Correct answer: C.
S-cam drum brakes are the most common foundation brakes on heavy commercial vehicles.
Question 18 of 25 · Emergency Maneuvers
Stab braking includes which of these steps?
-
A.
Apply brakes fully, release when wheels lock, reapply when wheels begin rolling
-
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 19 of 25 · Gauges
What gauge tells you the air pressure being delivered to the brakes?
-
A.
Application pressure gauge
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B.
Supply pressure gauge
-
C.
Speedometer
-
D.
Tachometer
Correct answer: A.
The application gauge shows pressure being applied to the brakes when the pedal is pressed.
Question 20 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
-
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 21 of 25 · Inspection
How can you check the slack adjusters?
-
A.
Pull hard on each slack adjuster — they should not move more than about 1 inch
-
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 22 of 25 · Combination Air
What does the trailer hand valve do?
-
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 23 of 25 · Combination Air
When a tractor protection valve closes, it:
-
A.
Opens the air supply to the trailer
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B.
Stops air from going to the trailer brakes
-
C.
Causes the parking brake to release
-
D.
Has no effect on the trailer
Correct answer: B.
When closed, it stops airflow to the trailer and applies trailer emergency brakes.
Question 24 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.
Question 25 of 25 · Combination Air
What component prevents air pressure loss in case of a leak between the tractor and trailer?
-
A.
Tractor protection valve
-
B.
Air dryer
-
C.
Foot brake valve
-
D.
Modulating valve
Correct answer: A.
The tractor protection valve closes off the trailer air supply during a leak, protecting tractor air.
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 New Hampshire Division of Motor Vehicles administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your New Hampshire CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Combination Air: 4 questions
- Inspection: 4 questions
- Foundation Brakes: 3 questions
- Compressor: 3 questions
- Tanks: 2 questions
- Spring Brakes: 2 questions
- System Basics: 1 question
- Bobtail: 1 question
- Operation: 1 question
- Parking: 1 question
- Speed Management: 1 question
- Emergency Maneuvers: 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 New Hampshire 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.