25 realistic Air Brakes practice questions for Minnesota CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 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 2 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 3 of 25 · Speed Management
Total stopping distance for an air-braked vehicle is made up of:
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A.
Perception distance + reaction distance + brake lag distance + braking distance
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B.
Just the braking distance
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C.
Reaction distance only
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D.
Brake lag distance only
Correct answer: A.
It is the sum of perception distance, reaction distance, brake lag distance, and braking distance.
Question 4 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 5 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 6 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 7 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 8 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 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
How do you know that you have an air leak?
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A.
Pressure drops faster than allowable rate
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B.
You hear hissing
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C.
Both A and B
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D.
The brake pedal feels firm
Correct answer: C.
Pressure dropping faster than allowed and audible hissing both indicate leaks.
Question 11 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 12 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 13 of 25 · Tanks
Why must air tanks be drained?
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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 14 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 15 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 16 of 25 · Spring Brakes
Modulating control valves do what?
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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.
Question 17 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.
Question 18 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 19 of 25 · System Basics
Air brake systems combine three different braking systems. They are:
-
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 20 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 21 of 25 · Tanks
When should you drain the air tanks?
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A.
Once a week
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B.
Only when the warning light is on
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C.
At the end of each working day
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D.
Once a month
Correct answer: C.
Drain tanks completely at the end of each working day to remove water and oil.
Question 22 of 25 · Spring Brakes
If your vehicle has dual parking control valves, you can use pressure from a separate tank to:
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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 23 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 24 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 25 of 25 · Speed Management
Air brake lag is:
-
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.
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 Minnesota Driver and Vehicle Services administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Minnesota CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Spring Brakes: 5 questions
- System Basics: 3 questions
- Inspection: 3 questions
- Speed Management: 2 questions
- Combination Air: 2 questions
- Gauges: 2 questions
- Tanks: 2 questions
- Operation: 1 question
- ABS: 1 question
- Parking: 1 question
- Compressor: 1 question
- Foundation Brakes: 1 question
- Bobtail: 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 Minnesota 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.