25 realistic Air Brakes practice questions for Ohio 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?
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A.
Brake pedal must be pumped to release
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B.
More air goes to the brakes; less foot pressure releases air
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C.
It locks the brakes permanently
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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 · 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 3 of 25 · Speed Management
Brake fade results from:
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A.
Oversized brakes
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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 4 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 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 · Combination Air
When a tractor protection valve closes, it:
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A.
Opens the air supply to the trailer
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B.
Stops air from going to the trailer brakes
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C.
Causes the parking brake to release
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D.
Has no effect on the trailer
Correct answer: B.
When closed, it stops airflow to the trailer and applies trailer emergency brakes.
Question 7 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 8 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 9 of 25 · Emergency Maneuvers
Stab braking includes which of these steps?
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A.
Apply brakes fully, release when wheels lock, reapply when wheels begin rolling
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B.
Apply brakes lightly until they fade
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C.
Pump the brakes rapidly
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D.
Apply the parking brake repeatedly
Correct answer: A.
Stab braking: full application, release on lockup, reapply when wheels turn again.
Question 10 of 25 · System Basics
Air brake systems combine three different braking systems. They are:
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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 11 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 12 of 25 · Spring Brakes
What does a "spring brake" mean?
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A.
A brake that operates only in spring weather
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B.
A mechanical brake that uses powerful springs to apply the brake when air pressure is lost
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C.
A specific kind of disc brake
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D.
A backup hydraulic brake
Correct answer: B.
Spring brakes use powerful coil springs to apply the brakes when air pressure is lost.
Question 13 of 25 · Inspection
How do you test the low pressure warning signal?
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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 14 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 15 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 16 of 25 · Inspection
When should you check the parking brake?
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A.
Whenever you start a trip
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B.
Once a month
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C.
Only at inspections
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D.
When the brake warning light is on
Correct answer: A.
Test the parking brake every trip — apply the brake and try to move forward gently.
Question 17 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 18 of 25 · Gauges
The supply pressure gauge tells you:
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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 19 of 25 · Parking
On a tractor with no trailer attached, the tractor parking brake control:
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A.
Will lock and unlock the front brakes
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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 20 of 25 · Spring Brakes
Spring brakes:
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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.
Question 21 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 22 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 23 of 25 · ABS
What is the proper procedure for an air brake stop in a vehicle equipped with ABS?
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A.
Pump the brakes
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B.
Press the brake pedal firmly and steadily; let ABS work
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C.
Disengage ABS first
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D.
Apply the parking brake
Correct answer: B.
With ABS, brake firmly and steadily. ABS prevents wheel lockup automatically.
Question 24 of 25 · Spring Brakes
What does the term "modulating control valve" refer to?
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A.
A valve to regulate engine RPM
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B.
A hand-operated valve that gradually applies the spring brakes
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C.
A device that drains air tanks
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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 · Compressor
What is the purpose of an alcohol evaporator?
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A.
Reduce alcohol in the driver's system
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B.
Reduce risk of frozen brake valves and lines in cold weather
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C.
Improve fuel economy
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D.
Lubricate the air compressor
Correct answer: B.
Alcohol injected into the air system reduces freeze-up risk in cold conditions.
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 Ohio Bureau of Motor Vehicles administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Ohio CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Inspection: 5 questions
- Spring Brakes: 4 questions
- Compressor: 2 questions
- Combination Air: 2 questions
- Parking: 2 questions
- System Basics: 2 questions
- Tanks: 2 questions
- Operation: 1 question
- Speed Management: 1 question
- Emergency Maneuvers: 1 question
- Warning Devices: 1 question
- Gauges: 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 Ohio 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.