Free Maryland DMV CDL — Air Brakes Practice Test 2026
The Maryland CDL — Air Brakes knowledge test has ~25 questions, and you need ~20 correct (80%) to pass. Practice free with questions written from the official Maryland driver handbook — every one has a clear explanation, and you can retake as often as you like with no sign-up.
Content reviewed against official driver handbooks · August 12, 2026
What's on the Maryland written test
The Maryland CDL — Air Brakes knowledge test has ~25 questions; you need ~20 correct (80%) to pass.
Based on the official Maryland exam format — always confirm current details with your state DMV.
Practice by topic
Focus on one area at a time — each topic has its own set of Maryland practice questions.
How to pass the first time
- Read the official Maryland driver handbook at least once before test day.
- Practice by topic to find your weak spots, then fix them.
- Take full-length practice tests until you pass comfortably every time.
- Read every explanation — understanding beats memorizing.
Maryland CDL — Air Brakes test — frequently asked questions
How many questions are on the Maryland CDL — Air Brakes test?
The Maryland CDL — Air Brakes knowledge test has ~25 questions, and you need ~20 correct (80%) to pass. Our practice tests mirror that format.
How many questions can I miss on the Maryland CDL — Air Brakes test?
You can miss up to ~5 of the ~25 questions and still pass — you need ~20 correct, which is 80%.
Can I retake the practice test?
Yes — practice as often as you like, completely free. The questions and answer choices are shuffled each time.
Can I take the Maryland practice test in Spanish?
Yes. Every question here is available in English and Spanish — switch languages anytime. Many states also offer the real knowledge test in languages other than English; confirm what Maryland offers with your DMV.
Can I practice on my phone?
Yes. The practice tests work in any mobile or desktop browser — no app or sign-up needed — so you can study anywhere.
Are these the real DMV test questions?
No. Our questions are original, written from the current official Maryland driver handbook to match the real test's topics and style. We are not affiliated with any DMV or government agency.
What's the best way to study?
Read the Maryland driver handbook, practice by topic to target weak areas, and finish with full-length tests until you consistently pass.
Other Maryland practice tests
Take the CDL — Air Brakes test in another state
Every Maryland question is written from the official Maryland driver handbook and checked against its current edition. DMV Test Free is a free, independent study resource — not affiliated with any DMV or government agency. About DMV Test Free
Maryland CDL — Air Brakes questions with answers
20 sample questions from the Maryland CDL — Air Brakes question bank — each with the correct answer, a short explanation, and the section of the official handbook it comes from.
Show all 20 questions with answers
1. When going down a long, steep grade, what is the role of the brakes according to the proper braking technique?
Correct answer: To assist the engine's own braking effect
On a long or steep downgrade the brakes serve only to back up the engine's braking effect; once you are in the proper low gear, you brake to feel a firm slowdown, release for a few seconds, and repeat.
2. What does the governor control on an air brake vehicle?
Correct answer: When the compressor pumps air into the storage tanks
The governor manages when the compressor loads air into the tanks and when it stops, keeping tank pressure within its working range.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.2 - Air Compressor Governor
3. On a tractor or straight truck, the spring brakes come fully on when air pressure drops into roughly what range?
Correct answer: 20 to 45 psi
Spring brakes on tractors and straight trucks apply fully once air pressure falls into the 20 to 45 psi band (often 20 to 30 psi).
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.14 - Spring Brakes
4. In which situations should you NOT use the parking brakes?
Correct answer: When the brakes are very hot, or very wet in freezing temperatures
Skip the parking brakes when the brakes are very hot from coming down a steep grade, or when they are very wet in freezing temperatures, since heat can damage them and freezing can lock the vehicle in place; use wheel chocks instead.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.4.8 - Parking Brakes
5. Truck tractors built with air brakes on or after which date are required to have antilock brakes?
Correct answer: March 1, 1997
Truck tractors equipped with air brakes and made from March 1, 1997 onward are required to carry antilock brakes; most other air brake vehicles produced from March 1, 1998 onward must have them too.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.15 - Antilock Braking Systems (ABS)
6. What is brake lag, the extra delay that air brakes add to stopping?
Correct answer: The short delay while air travels through the lines to the brakes
Brake lag is the time it takes for the brakes to react after the pedal is pushed; unlike hydraulic brakes that act instantly, air brakes need roughly a half second, sometimes longer, before the air reaches them through the lines.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.4.4 - Stopping Distance
7. What is the function of the stop light switch on an air brake vehicle?
Correct answer: It turns on the brake lights when you apply the brakes
The stop light switch uses air pressure to close an electrical circuit that turns on the brake lights whenever you apply the air brakes, warning drivers behind you.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.12 - Stop Light Switch
8. When the spring brakes finally set on a lightly loaded rig, or on a slick road surface, what extra danger appears?
Correct answer: The vehicle can slide and you may lose control of it
With little weight pressing the wheels down, or with poor traction beneath them, the sudden grab of the springs can throw the rig into a slide, which is one more reason to park before the tanks ever get that low.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.4.7 - Low Air Pressure
9. On newer air brake vehicles, how do you set the parking brakes?
Correct answer: Pull out the yellow, diamond-shaped knob
Newer vehicles use a yellow, diamond-shaped push-pull control knob; you pull it out to apply the parking (spring) brakes and push it in to release them.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.15 - Parking Brake Controls
10. On the older vehicles that have a front brake limiting valve, why should you leave the control in the 'normal' position?
Correct answer: So you keep full stopping power, since front braking is good in all conditions
The 'slippery' setting cuts front brake pressure in half and reduces stopping power; testing shows front braking works well in every condition and the front tires are unlikely to slide, so keeping the control on 'normal' preserves full stopping power.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.13 - Front Brake Limiting Valve
11. At 55 mph on dry pavement, about how long is the total stopping distance for an average driver with good brakes?
Correct answer: Over 450 feet
Because air brakes add roughly 32 feet of brake lag distance at 55 mph, the total stopping distance at that speed under good conditions comes to over 450 feet.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.4.4 - Stopping Distance
12. How does a modulating control valve let a driver use the spring brakes?
Correct answer: The farther you move the lever, the more firmly the spring brakes engage
A modulating valve lets you apply the spring brakes gradually from a spring-loaded dashboard handle; the more you move the lever, the more firmly the spring brakes engage, which helps if the service brakes fail.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.15 - Modulating Control Valves
13. What is physically going wrong inside a drum brake when too much heat makes the brakes fade?
Correct answer: The lining grips less and the drum swells, so the shoes reach farther and push with weaker force
Heat alters the lining chemically so it grips less, and it swells the drum outward, meaning the shoes must reach farther and then meet the drum with weaker force; keep it up and the rig may not slow at all.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.4.5 - Brake Fading or Failure
14. Why must you drain the water and oil out of the air tanks?
Correct answer: Because trapped water can freeze and lead to brake failure
Compressed air carries moisture and a little compressor oil that settle at the bottom of the tank; in freezing conditions that water can turn to ice and knock out the brakes, so empty the tanks fully after each driving day.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.4 - Air Tank Drains
15. Total stopping distance for an air brake vehicle is made up of which four parts?
Correct answer: Perception, reaction, brake lag, and braking distance
For air brake vehicles, total stopping distance equals perception distance plus reaction distance plus brake lag distance plus braking distance.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.4.4 - Stopping Distance
16. On a dual air brake vehicle, how much pressure should the compressor build in both systems before you drive?
Correct answer: A minimum of 100 psi in both the primary and secondary systems
You should let the compressor build a minimum of 100 psi in both the primary and secondary systems before driving away.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.2 - Dual Air Brake
17. On a vehicle with dual parking control valves and a separate air tank, what limits how you use it to release the spring brakes in an emergency?
Correct answer: The separate tank holds only enough air to release the brakes a few times
Pushing the control in draws air from a separate tank to free the spring brakes so you can move, but that tank holds only enough air to do this a few times, so you must plan the move carefully to avoid getting stranded in a dangerous spot.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.15 - Dual Parking Control Valves
18. What should you always do before leaving your vehicle unattended?
Correct answer: Apply the parking brakes or chock the wheels
Whenever you step away from the vehicle, set the parking brakes or chock the wheels; otherwise it could roll off and injure someone or cause damage.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.4.8 - Parking Brakes
19. On large buses, at about what pressure do the low air pressure warning devices usually signal?
Correct answer: About 80 to 85 psi
Most vehicles warn before 55 psi, but for large buses these warning devices normally trigger somewhere near 80 to 85 psi instead.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.1.11 - Low Air Pressure Warning
20. The antilock system on your truck quits working while you are out on the road. What does that mean for your braking?
Correct answer: Your ordinary brakes still work, so keep driving normally and get the system serviced soon
A failed antilock system does not take away the braking you already had; the truck stops the way it did before ABS existed, so drive as usual and have the fault repaired without delay.
Source: Maryland Commercial Driver License Manual (DL-151), Section 5.4.2 - Braking with Antilock Brakes