25 realistic Air Brakes practice questions for Oregon 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?
-
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 2 of 25 · Inspection
How much air pressure (psi) loss is allowed per minute for single vehicles with the engine off and brakes released?
-
A.
Up to 1 psi
-
B.
Up to 2 psi
-
C.
Up to 3 psi
-
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 3 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 4 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
-
B.
Just the braking distance
-
C.
Reaction distance only
-
D.
Brake lag distance only
Correct answer: A.
It is the sum of perception distance, reaction distance, brake lag distance, and braking distance.
Question 5 of 25 · System Basics
Air brakes use compressed air to:
-
A.
Lubricate the wheels
-
B.
Make the brakes work
-
C.
Cool the engine
-
D.
Increase fuel pressure
Correct answer: B.
Compressed air is the working fluid that applies the brakes in an air brake system.
Question 6 of 25 · Compressor
Which of these systems removes water and oil from compressed air?
-
A.
Air dryer
-
B.
Brake chamber
-
C.
Slack adjuster
-
D.
S-cam
Correct answer: A.
The air dryer removes moisture and oil from the air before it reaches the storage tanks.
Question 7 of 25 · Spring Brakes
Modulating control valves do what?
-
A.
Drain water from tanks
-
B.
Apply spring brakes gradually using a hand control
-
C.
Open the trailer supply line
-
D.
Switch to emergency brakes
Correct answer: B.
Modulating valves let you apply the spring brakes gradually rather than all at once.
Question 8 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
-
C.
Watch the brake light flash
-
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 9 of 25 · Inspection
How do you know that you have an air leak?
-
A.
Pressure drops faster than allowable rate
-
B.
You hear hissing
-
C.
Both A and B
-
D.
The brake pedal feels firm
Correct answer: C.
Pressure dropping faster than allowed and audible hissing both indicate leaks.
Question 10 of 25 · Foundation Brakes
Brake drums (or discs) must not have cracks longer than:
-
A.
1/4 the width of the friction area
-
B.
1/2 the width of the friction area
-
C.
3/4 the width of the friction area
-
D.
Anything is acceptable
Correct answer: B.
Cracks longer than one-half the width of the friction area are a safety violation.
Question 11 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
-
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 12 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 13 of 25 · Tanks
When should you drain the air tanks?
-
A.
Once a week
-
B.
Only when the warning light is on
-
C.
At the end of each working day
-
D.
Once a month
Correct answer: C.
Drain tanks completely at the end of each working day to remove water and oil.
Question 14 of 25 · Compressor
The air compressor governor controls:
-
A.
When the air compressor will pump air into the storage tanks
-
B.
When the brakes are applied
-
C.
When emergency brakes engage
-
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 15 of 25 · Inspection
How do you test the low pressure warning signal?
-
A.
With the engine off, pump the brake pedal until the signal activates (must come on at 60 psi or above)
-
B.
Drive the vehicle to lose pressure
-
C.
Open the air tank drain valve briefly
-
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 16 of 25 · Spring Brakes
What happens if the air pressure drops to between 20 and 45 psi?
-
A.
Nothing happens
-
B.
The compressor cuts in
-
C.
Spring brakes apply automatically
-
D.
The trailer is released
Correct answer: C.
Spring brakes typically apply automatically in this range, stopping the vehicle.
Question 17 of 25 · Combination Air
When should the trailer brake hand valve be used?
-
A.
To park the trailer
-
B.
To test the trailer brakes only
-
C.
To stop the entire combination
-
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 18 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 19 of 25 · Compressor
If the air compressor governor is not working properly:
-
A.
Air compressor may not maintain enough pressure to safely operate the brakes
-
B.
The brakes will work harder
-
C.
Brakes may not release at all
-
D.
There is no impact on safety
Correct answer: A.
A faulty governor can cause low system pressure or over-pressurization, both unsafe.
Question 20 of 25 · ABS
Which is correct about ABS-equipped vehicles?
-
A.
You should never use ABS
-
B.
ABS works only at high speeds
-
C.
You should drive the same way as without ABS, except brake firmly without pumping in emergencies
-
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 21 of 25 · Parking
When parking a tractor-trailer with spring brakes:
-
A.
Use the trailer hand valve only
-
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
-
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 22 of 25 · Combination Air
When a tractor protection valve closes, it:
-
A.
Opens the air supply to the trailer
-
B.
Stops air from going to the trailer brakes
-
C.
Causes the parking brake to release
-
D.
Has no effect on the trailer
Correct answer: B.
When closed, it stops airflow to the trailer and applies trailer emergency brakes.
Question 23 of 25 · Inspection
What is the air loss rate for combination vehicles with brakes applied?
-
A.
1 psi per minute
-
B.
2 psi per minute
-
C.
3 psi per minute
-
D.
4 psi per minute
Correct answer: D.
Combination vehicles can lose up to 4 psi per minute with brakes applied.
Question 24 of 25 · Spring Brakes
What does the term "modulating control valve" refer to?
-
A.
A valve to regulate engine RPM
-
B.
A hand-operated valve that gradually applies the spring brakes
-
C.
A device that drains air tanks
-
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 · Operation
When the brake pedal is pushed harder, what happens?
-
A.
Brake pedal must be pumped to release
-
B.
More air goes to the brakes; less foot pressure releases air
-
C.
It locks the brakes permanently
-
D.
The brakes do not respond
Correct answer: B.
The harder you push, the more air pressure is sent to the brake chambers.
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 Oregon Driver and Motor Vehicle Services administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Oregon CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Inspection: 7 questions
- Compressor: 3 questions
- Spring Brakes: 3 questions
- System Basics: 2 questions
- Parking: 2 questions
- Combination Air: 2 questions
- Speed Management: 1 question
- Foundation Brakes: 1 question
- Emergency Maneuvers: 1 question
- Tanks: 1 question
- ABS: 1 question
- Operation: 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 Oregon 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.