25 realistic Air Brakes practice questions for Washington CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 of 25 · Inspection
How much air pressure (psi) loss is allowed per minute for combination 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.
Combination vehicles allow up to 3 psi loss per minute with brakes released; with brakes applied, up to 4 psi.
Question 2 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 3 of 25 · ABS
If the ABS warning light comes on, you should:
-
A.
Stop driving immediately
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B.
Continue, knowing brakes still work — but ABS may not
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C.
Pump the brakes constantly
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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 4 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 5 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 6 of 25 · Inspection
How do you know that you have an air leak?
-
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 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 · 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 9 of 25 · Combination Air
When a tractor protection valve closes, it:
-
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 10 of 25 · Parking
Why should you use chocks?
-
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 11 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 12 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 13 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 14 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 15 of 25 · Inspection
When should you push the brake pedal during the air brake test?
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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 16 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 17 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 18 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 19 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 20 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 21 of 25 · Gauges
What gauge tells you the air pressure being delivered to the brakes?
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A.
Application pressure gauge
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B.
Supply pressure gauge
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C.
Speedometer
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D.
Tachometer
Correct answer: A.
The application gauge shows pressure being applied to the brakes when the pedal is pressed.
Question 22 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 23 of 25 · Combination Air
What does the trailer hand valve do?
-
A.
Operates only the trailer brakes
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B.
Operates both the tractor and trailer brakes
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C.
Engages the parking brakes
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D.
Lowers the landing gear
Correct answer: A.
The hand valve (trolley valve) controls only the trailer service brakes.
Question 24 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 25 of 25 · Speed Management
Why is it important not to over-apply the brakes?
-
A.
Brakes can overheat and fade
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B.
Brakes will lock and skid
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C.
You may lose control of the vehicle
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D.
All of the above
Correct answer: D.
Over-application leads to fade, lockup, skidding, and loss of control.
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 Washington Department of Licensing administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Washington CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Inspection: 5 questions
- Combination Air: 3 questions
- ABS: 2 questions
- Gauges: 2 questions
- Tanks: 2 questions
- Spring Brakes: 2 questions
- Parking: 2 questions
- System Basics: 2 questions
- Speed Management: 2 questions
- Foundation Brakes: 1 question
- Operation: 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 Washington 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.