25 realistic Air Brakes practice questions for Texas CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 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
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C.
Has no function
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D.
Operates only the trailer brakes
Correct answer: B.
The yellow diamond control sets and releases the tractor parking (spring) brakes.
Question 2 of 25 · Inspection
Air leaks during a static brake check could indicate:
-
A.
Faulty brake chambers or air lines
-
B.
Engine problems
-
C.
Bad tires
-
D.
Low fuel pressure
Correct answer: A.
Air leaks usually indicate faulty chambers, lines, fittings, or valves.
Question 3 of 25 · Compressor
What is the purpose of a one-way check valve?
-
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 4 of 25 · Spring Brakes
Where are spring brakes typically NOT found?
-
A.
Front steering axle
-
B.
Drive axles
-
C.
Trailer axles
-
D.
Tractor rear axles
Correct answer: A.
Spring brakes are found on drive axles and trailer axles, not normally on the steering axle.
Question 5 of 25 · ABS
If the ABS warning light comes on, you should:
-
A.
Stop driving immediately
-
B.
Continue, knowing brakes still work — but ABS may not
-
C.
Pump the brakes constantly
-
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 6 of 25 · Parking
When parking a tractor-trailer with spring brakes:
-
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 7 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 8 of 25 · System Basics
Which of the following is NOT in the air brake system?
-
A.
Brake chambers
-
B.
Slack adjusters
-
C.
S-cam
-
D.
Hydraulic master cylinder
Correct answer: D.
Hydraulic master cylinders are part of hydraulic systems, not air brake systems.
Question 9 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
-
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 10 of 25 · Speed Management
Why is it important not to over-apply the brakes?
-
A.
Brakes can overheat and fade
-
B.
Brakes will lock and skid
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C.
You may lose control of the vehicle
-
D.
All of the above
Correct answer: D.
Over-application leads to fade, lockup, skidding, and loss of control.
Question 11 of 25 · Spring Brakes
Spring brakes will come on automatically when air pressure drops to:
-
A.
100 psi
-
B.
60 psi or less
-
C.
20 to 45 psi
-
D.
10 psi
Correct answer: C.
Spring brakes typically apply automatically between 20 and 45 psi.
Question 12 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
-
D.
Apply the parking brake repeatedly
Correct answer: A.
Stab braking: full application, release on lockup, reapply when wheels turn again.
Question 13 of 25 · Warning Devices
The low air pressure warning device must come on when air pressure in the service tanks falls below:
-
A.
80 psi
-
B.
60 psi
-
C.
50 psi or 1/2 the compressor governor cut-out pressure
-
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 14 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 15 of 25 · System Basics
Which of these is NOT a part of the basic air brake system?
-
A.
Air compressor
-
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 16 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 17 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
-
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 18 of 25 · Bobtail
What is bobtail braking?
-
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 19 of 25 · Parking
Why should you use chocks?
-
A.
When parking on a hill
-
B.
On level ground when leaving the vehicle
-
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 20 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
-
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 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
-
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 · System Basics
Air brake systems combine three different braking systems. They are:
-
A.
Service, parking, and emergency brakes
-
B.
Hydraulic, parking, and emergency brakes
-
C.
Front, rear, and trailer brakes
-
D.
Disc, drum, and parking brakes
Correct answer: A.
The three are: the service brake, the parking brake, and the emergency brake.
Question 23 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 24 of 25 · Gauges
The supply pressure gauge tells you:
-
A.
The pressure of brake fluid
-
B.
How much air pressure is in the air tanks
-
C.
The trailer brake pressure
-
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 · Spring Brakes
Spring brakes:
-
A.
Are released by air pressure
-
B.
Are applied by air pressure
-
C.
Should be tested by pumping the brake pedal
-
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 Texas Department of Public Safety — Driver License Division administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Texas CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Compressor: 4 questions
- Spring Brakes: 4 questions
- Parking: 3 questions
- System Basics: 3 questions
- Inspection: 2 questions
- Speed Management: 2 questions
- Gauges: 2 questions
- ABS: 1 question
- Emergency Maneuvers: 1 question
- Warning Devices: 1 question
- Bobtail: 1 question
- Foundation Brakes: 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 Texas 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.