Free North Carolina DMV CDL — Air Brakes Practice Test 2026
The North Carolina CDL — Air Brakes knowledge test has ~25 questions, and you need ~20 correct (80%) to pass. Practice free with questions written from the official North Carolina 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 North Carolina written test
The North Carolina CDL — Air Brakes knowledge test has ~25 questions; you need ~20 correct (80%) to pass.
Based on the official North Carolina 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 North Carolina practice questions.
How to pass the first time
- Read the official North Carolina 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.
North Carolina CDL — Air Brakes test — frequently asked questions
How many questions are on the North Carolina CDL — Air Brakes test?
The North Carolina 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 North Carolina 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 North Carolina 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 North Carolina 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 North Carolina 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 North Carolina driver handbook, practice by topic to target weak areas, and finish with full-length tests until you consistently pass.
Other North Carolina practice tests
Take the CDL — Air Brakes test in another state
Every North Carolina question is written from the official North Carolina 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
North Carolina CDL — Air Brakes questions with answers
20 sample questions from the North Carolina 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. According to the manual, what is the most common brake problem found during roadside inspections?
Correct answer: Brakes that are out of adjustment
Out-of-adjustment brakes are cited as the most common issue turned up in roadside inspections, and too much slack makes a vehicle very hard to stop.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.3.2 Walk-around Inspection - Slack Adjusters
2. Compared with hydraulic brakes, air brakes add 'brake lag.' What is brake lag?
Correct answer: The short time for air to travel the lines and the brakes to act after you press the pedal
Unlike hydraulic brakes, which act at once, air brakes need a moment (about half a second or more) for the air to reach the brakes, adding lag to your stop.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.4.4 Stopping Distance
3. Why should you avoid pressing the brake pedal while the spring brakes are applied?
Correct answer: The combined spring and air forces could damage the brakes
Adding pedal-applied air pressure on top of the spring force can overload and damage the brakes, so it is best to never do it.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.15 Parking Brake Controls - Caution
4. What happens if a major leak lets all the air out of the brake system?
Correct answer: The springs apply the brakes
Because the springs are normally held off by air pressure, losing all of the system's air lets them snap on and brake the vehicle.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.14 Spring Brakes
5. What should you conclude if the safety relief valve lets air escape?
Correct answer: Something is wrong and a mechanic should correct it
The relief valve venting is a sign of a fault in the system, so the problem should be diagnosed and repaired by a mechanic.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.6 Safety Valve
6. 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 brakes the total is perception distance plus reaction distance plus brake-lag distance plus braking distance.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.4.4 Stopping Distance
7. How do you test that the parking brake will hold?
Correct answer: Set the parking brake, then gently try to pull forward in a low gear
With the parking brake set, ease against it in a low gear; if it holds the vehicle in place, the parking brake passed the test.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.3.3 Final Air Brake Check - Test Parking Brake
8. When the low-air warning first lights up while driving, what should you do?
Correct answer: Come to a controlled, safe stop immediately, before you lose the ability to brake
Don't wait for the spring brakes to grab on their own; stop safely as soon as the warning appears, while braking is still under your control.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.14 Spring Brakes
9. With the engine off and the foot brake fully applied for one minute, the air-loss rate is too high on a single vehicle if pressure drops more than:
Correct answer: 3 psi in one minute
Holding the brake applied, a single vehicle should not lose more than 3 psi per minute (a combination vehicle no more than 4 psi); more than that means find and fix the leak.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.3.3 Final Air Brake Check - Air Leakage Rate
10. On a long, steep downgrade, the manual describes the use of the brakes as:
Correct answer: A backup to the engine's own braking, applied only after you have shifted into a low gear
Descending, you first pick the proper low gear and let the engine do most of the work; the brakes are used in snubs only to back up that engine braking.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.4.6 Proper Braking Technique
11. On newer air-brake vehicles, how do you set the parking (spring) brakes?
Correct answer: Pull out the yellow, diamond-shaped knob
Newer vehicles use a yellow, diamond-shaped push-pull knob: pull it out to apply the parking brakes and push it in to release them.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.15 Parking Brake Controls
12. While you drive, what holds the powerful spring brakes in the released position?
Correct answer: Air pressure in the system
Air pressure compresses and holds back the springs while driving; if that air escapes, the springs apply the brakes.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.14 Spring Brakes
13. What does the supply pressure gauge tell the driver?
Correct answer: How much air pressure is currently stored in the tanks
Every air-brake vehicle has a supply gauge showing the pressure held in the reservoirs; a dual system shows both halves.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.9 Supply Pressure Gauges
14. If one circuit of a dual air brake system loses most of its pressure, what should you expect?
Correct answer: Either the front or rear brakes lose power, so stopping takes longer
With one circuit low, one set of brakes (front or rear) will not do its full share, so your stopping distance grows; stop safely and get it repaired.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.2 Dual Air Brake Systems
15. If your tanks have manually operated drain valves rather than automatic ones, how often should you drain them?
Correct answer: At the end of each driving day
Manual drains have to be opened by the driver, and this should be done at the close of every day of driving to clear out accumulated water and oil.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.4 Air Tank Drains
16. On a tractor or straight truck, the spring brakes come fully on when air pressure falls into roughly what range?
Correct answer: 20 to 45 psi
As pressure bleeds down to about 20 to 45 psi (often 20 to 30), the springs fully apply, so you should stop while you still have air to control the brakes.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.14 Spring Brakes
17. During a walk-around, brake linings should be rejected if they are:
Correct answer: Loose, soaked with oil or grease, or dangerously thin
Linings must be secure and free of oil or grease, and they cannot be worn dangerously thin; the mechanical parts must all be present and sound.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.3.2 Walk-around Inspection - Brake Drums, Linings, Hoses
18. How should you test the service brakes at the end of the brake check?
Correct answer: Roll forward slowly, about 5 mph, then press firmly and note any pulling or delay
Move ahead at about 5 mph and apply the brakes firmly; watch for the vehicle pulling to one side, an odd feel, or a slow response.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.3.3 Final Air Brake Check - Test Service Brakes
19. A visible low-air-pressure warning must come on before tank pressure drops below what level?
Correct answer: Below 60 psi
Air-brake vehicles must have a warning you can see that activates before pressure falls under 60 psi (or half the cut-out pressure on some older trucks).
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.11 Low Air Pressure Warning
20. Truck tractors with air brakes are required to have ABS if they were built on or after which date?
Correct answer: March 1, 1997
The rule requires ABS on air-brake truck tractors made on or after March 1, 1997; most other air-brake vehicles from March 1, 1998 onward.
Source: North Carolina Commercial Driver License Manual, Section 5 (Air Brakes), 5.1.16 Antilock Braking Systems (ABS)