25 realistic Air Brakes practice questions for Pennsylvania CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 of 25 · Inspection
How do you test the low pressure warning signal?
-
A.
With the engine off, pump the brake pedal until the signal activates (must come on at 60 psi or above)
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B.
Drive the vehicle to lose pressure
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C.
Open the air tank drain valve briefly
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D.
Push the parking brake control
Correct answer: A.
Pump the brake to lower pressure; warning must come on at 60 psi or higher.
Question 2 of 25 · Warning Devices
The low air pressure warning device must come on when air pressure in the service tanks falls below:
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A.
80 psi
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B.
60 psi
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C.
50 psi or 1/2 the compressor governor cut-out pressure
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D.
20 psi
Correct answer: C.
Low pressure warnings activate at 60 psi or less (or half the cut-out pressure on older vehicles).
Question 3 of 25 · Gauges
What gauge tells you the air pressure being delivered to the brakes?
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A.
Application pressure gauge
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B.
Supply pressure gauge
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C.
Speedometer
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D.
Tachometer
Correct answer: A.
The application gauge shows pressure being applied to the brakes when the pedal is pressed.
Question 4 of 25 · System Basics
Which of the following is NOT in the air brake system?
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A.
Brake chambers
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B.
Slack adjusters
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C.
S-cam
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D.
Hydraulic master cylinder
Correct answer: D.
Hydraulic master cylinders are part of hydraulic systems, not air brake systems.
Question 5 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 6 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 7 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 8 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 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 · 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 11 of 25 · Speed Management
Which of these statements is true about brake fading?
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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 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
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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 13 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 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
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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 15 of 25 · Spring Brakes
Spring brakes will come on automatically when air pressure drops to:
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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 · Speed Management
Air brake lag distance at 55 mph on dry pavement adds about:
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A.
12 feet to stopping distance
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B.
32 feet to stopping distance
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C.
50 feet to stopping distance
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D.
100 feet to stopping distance
Correct answer: B.
Air brake systems have a lag of about 0.4 seconds, adding roughly 32 feet at 55 mph.
Question 17 of 25 · Inspection
What is the air loss rate for combination vehicles with brakes applied?
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A.
1 psi per minute
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B.
2 psi per minute
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C.
3 psi per minute
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D.
4 psi per minute
Correct answer: D.
Combination vehicles can lose up to 4 psi per minute with brakes applied.
Question 18 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 19 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 20 of 25 · Foundation Brakes
What's the most common type of foundation brake found on heavy vehicles?
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A.
Disc brakes
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B.
Wedge drum brakes
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C.
S-cam drum brakes
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D.
Air-over-hydraulic
Correct answer: C.
S-cam drum brakes are the most common foundation brakes on heavy commercial vehicles.
Question 21 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 22 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 23 of 25 · Compressor
What is the purpose of a one-way check valve?
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A.
Allow air to flow in one direction
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B.
Allow water to drain
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C.
Prevent the trailer from moving
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D.
Increase pressure in the supply tank
Correct answer: A.
It permits airflow in one direction only — protecting the system if the compressor fails.
Question 24 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 25 of 25 · Spring Brakes
Modulating control valves do what?
-
A.
Drain water from tanks
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B.
Apply spring brakes gradually using a hand control
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C.
Open the trailer supply line
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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 Pennsylvania Department of Transportation — Driver Services administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Pennsylvania CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Inspection: 7 questions
- Compressor: 3 questions
- Spring Brakes: 3 questions
- System Basics: 2 questions
- Combination Air: 2 questions
- Speed Management: 2 questions
- Foundation Brakes: 2 questions
- Warning Devices: 1 question
- Gauges: 1 question
- Parking: 1 question
- ABS: 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 Pennsylvania 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.