25 realistic Air Brakes practice questions for District of Columbia CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 of 25 · Combination Air
What component prevents air pressure loss in case of a leak between the tractor and trailer?
-
A.
Tractor protection valve
-
B.
Air dryer
-
C.
Foot brake valve
-
D.
Modulating valve
Correct answer: A.
The tractor protection valve closes off the trailer air supply during a leak, protecting tractor air.
Question 2 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 3 of 25 · Inspection
When should you check the parking brake?
-
A.
Whenever you start a trip
-
B.
Once a month
-
C.
Only at inspections
-
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 4 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 5 of 25 · Spring Brakes
What does a "spring brake" mean?
-
A.
A brake that operates only in spring weather
-
B.
A mechanical brake that uses powerful springs to apply the brake when air pressure is lost
-
C.
A specific kind of disc brake
-
D.
A backup hydraulic brake
Correct answer: B.
Spring brakes use powerful coil springs to apply the brakes when air pressure is lost.
Question 6 of 25 · Parking
You should never use the trailer hand valve to park because:
-
A.
It is not strong enough
-
B.
The air could leak out, releasing the brakes
-
C.
It only applies the front brakes
-
D.
It is not connected to the trailer
Correct answer: B.
If air leaks out, the brakes release. Always use the parking brake to park.
Question 7 of 25 · Foundation Brakes
When checking the slack adjuster on S-cam brakes, the slack adjuster should not move more than:
-
A.
1/2 inch
-
B.
1 inch
-
C.
About 1 inch where the push rod attaches to it
-
D.
2 inches
Correct answer: C.
When pulled by hand, the slack adjuster shouldn't move more than about an inch where the push rod attaches.
Question 8 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 9 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 10 of 25 · Emergency Maneuvers
Which of these statements is true about emergency braking?
-
A.
Pumping the brakes is always best
-
B.
Stab braking is for non-ABS vehicles in emergencies
-
C.
You should brake harder than necessary
-
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 11 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
-
C.
Has no function
-
D.
Operates only the trailer brakes
Correct answer: B.
The yellow diamond control sets and releases the tractor parking (spring) brakes.
Question 12 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 13 of 25 · System Basics
Which of these is NOT a part of the basic air brake system?
-
A.
Air compressor
-
B.
Air storage tanks
-
C.
Hydraulic master cylinder
-
D.
Foot brake valve
Correct answer: C.
Hydraulic master cylinders belong to hydraulic brake systems, not air brake systems.
Question 14 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 15 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 16 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 17 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 18 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
-
C.
Disengage ABS first
-
D.
Apply the parking brake
Correct answer: B.
With ABS, brake firmly and steadily. ABS prevents wheel lockup automatically.
Question 19 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 20 of 25 · Inspection
When should you push the brake pedal during the air brake test?
-
A.
Only when the engine is running
-
B.
When checking the compressor
-
C.
When the engine is off, to check that pressure does not drop too quickly
-
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 21 of 25 · Compressor
What is the purpose of a one-way check valve?
-
A.
Allow air to flow in one direction
-
B.
Allow water to drain
-
C.
Prevent the trailer from moving
-
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 22 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 23 of 25 · Tanks
Air brake equipped vehicles must have:
-
A.
Only an emergency brake
-
B.
A drain valve in each air tank
-
C.
Hydraulic backup
-
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 24 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 25 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.
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 DC Department of Motor Vehicles administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your District of Columbia CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Inspection: 6 questions
- Foundation Brakes: 3 questions
- System Basics: 3 questions
- Combination Air: 2 questions
- Spring Brakes: 2 questions
- Parking: 2 questions
- Compressor: 2 questions
- Gauges: 1 question
- Emergency Maneuvers: 1 question
- ABS: 1 question
- Speed Management: 1 question
- Tanks: 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 District of Columbia 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.