25 realistic Air Brakes practice questions for Connecticut CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 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 2 of 25 · Inspection
How much air pressure (psi) loss is allowed per minute for combination 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.
Combination vehicles allow up to 3 psi loss per minute with brakes released; with brakes applied, up to 4 psi.
Question 3 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 4 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 5 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 6 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 7 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 8 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 9 of 25 · Spring Brakes
If your vehicle has dual parking control valves, you can use pressure from a separate tank to:
-
A.
Charge the trailer brakes
-
B.
Release the spring brakes to move a short distance if the main system loses air
-
C.
Operate the steering
-
D.
Power the air horn
Correct answer: B.
Dual parking control valves let you temporarily release spring brakes from a backup tank.
Question 10 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 11 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 12 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 13 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 14 of 25 · Operation
When making a normal stop, what should you do?
-
A.
Push the brake pedal down so you can stop at a safe place and remain in control
-
B.
Pump the brakes
-
C.
Use only the parking brake
-
D.
Use only the trailer hand valve
Correct answer: A.
Apply pressure progressively to come to a smooth, safe stop while maintaining control.
Question 15 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 16 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 17 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 18 of 25 · Combination Air
What does the trailer hand valve do?
-
A.
Operates only the trailer brakes
-
B.
Operates both the tractor and trailer brakes
-
C.
Engages the parking brakes
-
D.
Lowers the landing gear
Correct answer: A.
The hand valve (trolley valve) controls only the trailer service brakes.
Question 19 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 20 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 21 of 25 · Speed Management
Which of these statements is true about brake fading?
-
A.
Resting brakes does not bring back braking power
-
B.
Brake fade is caused by drums getting too hot
-
C.
Brake fade only affects new vehicles
-
D.
Brake fade only happens with disc brakes
Correct answer: B.
Brake fade results from overheated drums or discs reducing friction and braking ability.
Question 22 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 23 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 24 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 25 of 25 · Gauges
You should know your vehicle is properly equipped with which gauge?
-
A.
Two air pressure gauges (or one with two needles) showing pressure in primary and secondary tanks
-
B.
A vacuum gauge
-
C.
A voltage meter only
-
D.
A coolant gauge only
Correct answer: A.
Dual air systems require gauges for both primary and secondary air supplies.
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 Connecticut Department of Motor Vehicles administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Connecticut CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Spring Brakes: 4 questions
- Inspection: 4 questions
- Compressor: 3 questions
- Foundation Brakes: 3 questions
- Gauges: 3 questions
- System Basics: 2 questions
- Parking: 1 question
- Operation: 1 question
- ABS: 1 question
- Tanks: 1 question
- Combination Air: 1 question
- Speed Management: 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 Connecticut 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.