100+ Free CDL Air Brakes Practice Questions
Pass your CDL Air Brakes Knowledge Test exam on the first try — instant access, no signup required.
In cold weather, what should drivers monitor more closely in the air brake system?
Explore More CDL
Continue into nearby exams from the same family. Each card keeps practice questions, study guides, flashcards, videos, and articles in one place.
More From This Family
Videos and articles for deeper review.
Key Facts: CDL Air Brakes Exam
80%
Passing Score
FMCSA / State DMV
~25
Questions
Varies by state
60 psi
Low-Pressure Warning Threshold
FMCSA 49 CFR 393.51
2 / 3 psi/min
Static Leak Rate (single / combo)
FMCSA CDL Manual Section 5
L
Restriction Removed by Passing
FMCSA
150 psi
Safety Valve Pop-Off Setting
FMCSA CDL Manual Section 5
The CDL Air Brakes Knowledge Test requires 80% on roughly 25 multiple-choice questions covering FMCSA Model CDL Manual Section 5. Topics include the air brake system (compressor, governor 100-125 psi cut-in / 125-140 psi cut-out, wet and service tanks, air dryer, foot valve, brake chambers, S-cam, slack adjusters), dual air systems, spring parking brakes, anti-compounding, low-pressure warning at 60 psi, applied and static leak rates (2/3 psi single, 3/4 psi combination), and driving techniques including controlled braking, stab braking, brake fade, mountain descents, and ABS. Passing removes the L restriction.
Sample CDL Air Brakes Practice Questions
Try these sample questions to test your CDL Air Brakes exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.
1What component compresses air for the air brake system?
2What is the typical air compressor governor cut-out pressure range?
3At what pressure does the air compressor governor typically cut IN to resume pumping?
4What is the purpose of the air dryer in the brake system?
5What is the function of the wet tank (supply or service reservoir)?
6At what pressure does the safety valve in the supply tank typically open to release excess air?
7What is the role of the foot valve (also called the treadle or service brake valve)?
8What does the brake chamber do?
9What is the role of the slack adjuster on an S-cam brake?
10What does the S-cam do in an S-cam drum brake?
About the CDL Air Brakes Exam
The CDL Air Brakes Knowledge Test is a standalone written exam required to operate any commercial motor vehicle equipped with full air brakes or air-over-hydraulic brakes. Without passing this test, the L restriction is placed on the CDL, prohibiting operation of air-brake-equipped vehicles. The test is universal across all 50 states and DC and is drawn from FMCSA Model CDL Manual Section 5.
Questions
25 scored questions
Time Limit
60 minutes
Passing Score
80%
Exam Fee
$5-25 (State DMV / FMCSA)
CDL Air Brakes Exam Content Outline
Air Brake System Components
Compressor and governor cut-in/cut-out, wet and dry tanks, air dryer with desiccant, foot valve, brake chambers, S-cam, slack adjusters, safety valve (150 psi), dual air systems
Pre-Trip Air Brake Inspection
Build to governor cut-out, static leak rate (2 psi/min single, 3 psi/min combo), applied leak rate (3 psi/min single, 4 psi/min combo), low-pressure warning at 60 psi, slack adjuster travel
Driving with Air Brakes
Normal stops, controlled braking, stab braking, brake fade, downgrade strategy, engine retarders and Jake brakes, ABS behavior, never use parking brake while moving
Parking, Emergency, and Dual Systems
Spring (parking) brakes, yellow diamond parking control, red 8-sided trailer air supply, anti-compounding modulator, glad hands, dual primary and secondary system operation
How to Pass the CDL Air Brakes Exam
What You Need to Know
- Passing score: 80%
- Exam length: 25 questions
- Time limit: 60 minutes
- Exam fee: $5-25
Keys to Passing
- Complete 500+ practice questions
- Score 80%+ consistently before scheduling
- Focus on highest-weighted sections
- Use our AI tutor for tough concepts
CDL Air Brakes Study Tips from Top Performers
Frequently Asked Questions
How many questions are on the CDL Air Brakes Knowledge Test?
The CDL Air Brakes Knowledge Test typically has around 25 multiple-choice questions (exact count varies by state). You need 80% (usually 20 out of 25) to pass. The test draws from FMCSA Model CDL Manual Section 5 and is universal across all 50 states and DC.
What is the L restriction on a CDL?
The L restriction means 'no air brakes.' It is placed on a CDL when the driver either does not pass the Air Brakes Knowledge Test, does not perform the pre-trip air brake inspection in the skills test, or does not take the skills test in a vehicle equipped with full air brakes. With the L restriction, the driver is prohibited from operating any commercial motor vehicle equipped with air brakes.
At what pressure does the low air pressure warning activate?
FMCSA requires the low air pressure warning to activate at 60 psi or higher. The warning includes both a visual indicator (light or flag) and an audible alarm such as a buzzer. If the warning activates while driving, the driver should bring the vehicle to a safe stop immediately, because once pressure drops further the spring brakes will apply automatically.
What are the air leak rate limits during pre-trip inspection?
After building air to governor cut-out and shutting off the engine, the static leak rate (brakes released) should not exceed 2 psi/min for a single vehicle or 3 psi/min for a combination vehicle. With brakes applied, the applied leak rate should not exceed 3 psi/min for a single vehicle or 4 psi/min for a combination vehicle. Higher leak rates indicate a problem that must be repaired before driving.
What is the difference between service brakes and parking brakes on an air-brake CMV?
Service brakes are applied by pressing the foot valve (treadle), sending air to the brake chambers to push the pushrod and S-cam to apply the brake shoes. Parking brakes use spring brakes — strong springs that apply the brakes when air pressure is released. Air pressure holds the spring brakes off; if air pressure drops below about 20-45 psi, the spring brakes apply automatically. Never use the parking brake to slow a moving vehicle.
What is brake fade and how do you prevent it?
Brake fade is the loss of braking effectiveness from overheated brakes — drums expand away from the linings and the linings glaze. It typically happens on long downgrades when service brakes are used continuously. Prevent fade by selecting a low gear BEFORE descending, using engine and exhaust retarders, and applying brakes only briefly and firmly (pump-and-cool) to maintain a safe speed. Never ride the brakes.