25 realistic Air Brakes practice questions for Colorado CDL applicants. Required for: Vehicles equipped with air brake systems. Official test: 25 questions, 20 correct to pass.
Question 1 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 2 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 3 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 4 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 5 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 6 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 7 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 8 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 9 of 25 · Speed Management
Air brake lag distance at 55 mph on dry pavement adds about:
-
A.
12 feet to stopping distance
-
B.
32 feet to stopping distance
-
C.
50 feet to stopping distance
-
D.
100 feet to stopping distance
Correct answer: B.
Air brake systems have a lag of about 0.4 seconds, adding roughly 32 feet at 55 mph.
Question 10 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.
Question 11 of 25 · Tanks
What is the safety valve set to open at?
-
A.
100 psi
-
B.
125 psi
-
C.
150 psi
-
D.
180 psi
Correct answer: C.
The safety valve is normally set to open at 150 psi to protect the system from over-pressurization.
Question 12 of 25 · Warning Devices
The low air pressure warning device must come on when air pressure in the service tanks falls below:
-
A.
80 psi
-
B.
60 psi
-
C.
50 psi or 1/2 the compressor governor cut-out pressure
-
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 13 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 14 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 15 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 16 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 17 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 18 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 19 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 20 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 21 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 22 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 23 of 25 · Bobtail
What is bobtail braking?
-
A.
Braking with no trailer attached, when the tractor brakes harder than usual
-
B.
A type of trailer-only braking
-
C.
Emergency braking on doubles
-
D.
A way to test the parking brake
Correct answer: A.
A bobtail tractor (without a trailer) brakes hard because it has less weight on the drive axles.
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 · 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.
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 Colorado Division of Motor Vehicles administers this knowledge test as part of its commercial driver license program. To earn the Air Brakes Endorsement on your Colorado CDL, you need to score at least 20 out of 25 on the official version.
Topics covered on this practice test
- Inspection: 5 questions
- Gauges: 3 questions
- Compressor: 2 questions
- Foundation Brakes: 2 questions
- Speed Management: 2 questions
- Tanks: 2 questions
- System Basics: 2 questions
- ABS: 1 question
- Warning Devices: 1 question
- Combination Air: 1 question
- Emergency Maneuvers: 1 question
- Spring Brakes: 1 question
- Parking: 1 question
- Bobtail: 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 Colorado 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.