25 realistic Air Brakes practice questions for Wisconsin 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?
-
A.
Brake pedal must be pumped to release
-
B.
More air goes to the brakes; less foot pressure releases air
-
C.
It locks the brakes permanently
-
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 · Foundation Brakes
What's the most common type of foundation brake found on heavy vehicles?
-
A.
Disc brakes
-
B.
Wedge drum brakes
-
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 3 of 25 · Compressor
What is the purpose of an alcohol evaporator?
-
A.
Reduce alcohol in the driver's system
-
B.
Reduce risk of frozen brake valves and lines in cold weather
-
C.
Improve fuel economy
-
D.
Lubricate the air compressor
Correct answer: B.
Alcohol injected into the air system reduces freeze-up risk in cold conditions.
Question 4 of 25 · Speed Management
Which of these statements is true about brake fading?
-
A.
Resting brakes does not bring back braking power
-
B.
Brake fade is caused by drums getting too hot
-
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 5 of 25 · Compressor
The air compressor governor controls:
-
A.
When the air compressor will pump air into the storage tanks
-
B.
When the brakes are applied
-
C.
When emergency brakes engage
-
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 6 of 25 · Inspection
How much air pressure (psi) loss is allowed per minute for single vehicles with the engine off and brakes released?
-
A.
Up to 1 psi
-
B.
Up to 2 psi
-
C.
Up to 3 psi
-
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 7 of 25 · Foundation Brakes
Brake drums (or discs) must not have cracks longer than:
-
A.
1/4 the width of the friction area
-
B.
1/2 the width of the friction area
-
C.
3/4 the width of the friction area
-
D.
Anything is acceptable
Correct answer: B.
Cracks longer than one-half the width of the friction area are a safety violation.
Question 8 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)
-
B.
Drive the vehicle to lose pressure
-
C.
Open the air tank drain valve briefly
-
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 9 of 25 · Parking
On a tractor with no trailer attached, the tractor parking brake control:
-
A.
Will lock and unlock the front brakes
-
B.
Will set or release the spring brakes on the tractor
-
C.
Has no function
-
D.
Operates only the trailer brakes
Correct answer: B.
The yellow diamond control sets and releases the tractor parking (spring) brakes.
Question 10 of 25 · Speed Management
Brake fade results from:
-
A.
Oversized brakes
-
B.
Excessive heat from prolonged braking
-
C.
Too much air pressure
-
D.
Cold weather
Correct answer: B.
Brake fade is caused by overheating brakes that lose stopping power.
Question 11 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
-
B.
Up to 2 psi
-
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 12 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.
Question 13 of 25 · Speed Management
Air brake lag distance at 55 mph on dry pavement adds about:
-
A.
12 feet to stopping distance
-
B.
32 feet to stopping distance
-
C.
50 feet to stopping distance
-
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 14 of 25 · Spring Brakes
What happens if the air pressure drops to between 20 and 45 psi?
-
A.
Nothing happens
-
B.
The compressor cuts in
-
C.
Spring brakes apply automatically
-
D.
The trailer is released
Correct answer: C.
Spring brakes typically apply automatically in this range, stopping the vehicle.
Question 15 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 16 of 25 · Combination Air
What does the trailer hand valve do?
-
A.
Operates only the trailer brakes
-
B.
Operates both the tractor and trailer brakes
-
C.
Engages the parking brakes
-
D.
Lowers the landing gear
Correct answer: A.
The hand valve (trolley valve) controls only the trailer service brakes.
Question 17 of 25 · ABS
Which is correct about ABS-equipped vehicles?
-
A.
You should never use ABS
-
B.
ABS works only at high speeds
-
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 18 of 25 · Gauges
What gauge tells you the air pressure being delivered to the brakes?
-
A.
Application pressure gauge
-
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 19 of 25 · System Basics
Air brakes use compressed air to:
-
A.
Lubricate the wheels
-
B.
Make the brakes work
-
C.
Cool the engine
-
D.
Increase fuel pressure
Correct answer: B.
Compressed air is the working fluid that applies the brakes in an air brake system.
Question 20 of 25 · Operation
When making a normal stop, what should you do?
-
A.
Push the brake pedal down so you can stop at a safe place and remain in control
-
B.
Pump the brakes
-
C.
Use only the parking brake
-
D.
Use only the trailer hand valve
Correct answer: A.
Apply pressure progressively to come to a smooth, safe stop while maintaining control.
Question 21 of 25 · Tanks
What is the safety valve set to open at?
-
A.
100 psi
-
B.
125 psi
-
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 22 of 25 · Tanks
When should you drain the air tanks?
-
A.
Once a week
-
B.
Only when the warning light is on
-
C.
At the end of each working day
-
D.
Once a month
Correct answer: C.
Drain tanks completely at the end of each working day to remove water and oil.
Question 23 of 25 · Compressor
What is the purpose of a one-way check valve?
-
A.
Allow air to flow in one direction
-
B.
Allow water to drain
-
C.
Prevent the trailer from moving
-
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 · Spring Brakes
What does the term "modulating control valve" refer to?
-
A.
A valve to regulate engine RPM
-
B.
A hand-operated valve that gradually applies the spring brakes
-
C.
A device that drains air tanks
-
D.
The trailer hand valve
Correct answer: B.
A modulating control valve allows gradual application of spring brakes for emergency or backup use.
Question 25 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
-
C.
It only applies the front brakes
-
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.
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 Wisconsin Division of Motor Vehicles administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Wisconsin CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Compressor: 3 questions
- Speed Management: 3 questions
- Inspection: 3 questions
- Combination Air: 3 questions
- Operation: 2 questions
- Foundation Brakes: 2 questions
- Parking: 2 questions
- Spring Brakes: 2 questions
- Tanks: 2 questions
- ABS: 1 question
- Gauges: 1 question
- System Basics: 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 Wisconsin 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.