25 realistic Air Brakes practice questions for Tennessee CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 of 25 · Speed Management
Brake fade results from:
-
A.
Oversized brakes
-
B.
Excessive heat from prolonged braking
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C.
Too much air pressure
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D.
Cold weather
Correct answer: B.
Brake fade is caused by overheating brakes that lose stopping power.
Question 2 of 25 · Tanks
Why must air tanks be drained?
-
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 3 of 25 · Spring Brakes
Spring brakes will come on automatically when air pressure drops to:
-
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 4 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 5 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 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 · 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 8 of 25 · Combination Air
What component prevents air pressure loss in case of a leak between the tractor and trailer?
-
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 9 of 25 · Speed Management
Air brake lag distance at 55 mph on dry pavement adds about:
-
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 10 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 11 of 25 · Inspection
When should you push the brake pedal during the air brake test?
-
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 12 of 25 · ABS
Which is correct about ABS-equipped vehicles?
-
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 13 of 25 · Spring Brakes
If your vehicle has dual parking control valves, you can use pressure from a separate tank to:
-
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 14 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 15 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
-
D.
All of the above
Correct answer: D.
Chocks should be used when required to prevent vehicle movement.
Question 16 of 25 · Emergency Maneuvers
Which of these statements is true about emergency braking?
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A.
Pumping the brakes is always best
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B.
Stab braking is for non-ABS vehicles in emergencies
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C.
You should brake harder than necessary
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D.
Stab braking is the same as pumping
Correct answer: B.
Stab braking applies to non-ABS systems: full application until lockup, then release.
Question 17 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 18 of 25 · Speed Management
Total stopping distance for an air-braked vehicle is made up of:
-
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 19 of 25 · Tanks
Air brake equipped vehicles must have:
-
A.
Only an emergency brake
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B.
A drain valve in each air tank
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C.
Hydraulic backup
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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 20 of 25 · Inspection
Which of the following must be checked before operating an air-braked vehicle?
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A.
Low air pressure warning signal
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B.
Spring brakes set automatically
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C.
Air leakage rate
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D.
All of the above
Correct answer: D.
All of these are required pre-trip air brake checks.
Question 21 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 22 of 25 · Compressor
Brake-system air pressure should build from 85 to 100 psi within:
-
A.
15 seconds in dual systems
-
B.
30 seconds in dual systems
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C.
45 seconds in dual systems
-
D.
60 seconds in dual systems
Correct answer: C.
Build-up time should be within 45 seconds in dual air systems.
Question 23 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 24 of 25 · ABS
What is the proper procedure for an air brake stop in a vehicle equipped with ABS?
-
A.
Pump the brakes
-
B.
Press the brake pedal firmly and steadily; let ABS work
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C.
Disengage ABS first
-
D.
Apply the parking brake
Correct answer: B.
With ABS, brake firmly and steadily. ABS prevents wheel lockup automatically.
Question 25 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.
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 Tennessee Department of Safety — Driver Services administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Tennessee CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Inspection: 5 questions
- Compressor: 4 questions
- Speed Management: 3 questions
- Spring Brakes: 3 questions
- Tanks: 2 questions
- ABS: 2 questions
- Gauges: 1 question
- Combination Air: 1 question
- Foundation Brakes: 1 question
- Parking: 1 question
- Emergency Maneuvers: 1 question
- Warning Devices: 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 Tennessee 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.