Alabama CDL — Air Brakes Practice Test
The Alabama CDL — Air Brakes knowledge test has 25 questions, and you need 20 correct (80%) to pass. Practice free with questions written from the official Alabama 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 · July 7, 2026
What's on the Alabama written test
The Alabama CDL — Air Brakes knowledge test has 25 questions; you need 20 correct (80%) to pass.
Based on the official Alabama 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 Alabama practice questions.
Following distance, scanning, blind spots, using your lights, and the habits that prevent crashes and keep you in control.
Legal and illegal parking, colored curbs, distances from hydrants and crossings, and how to park safely on hills.
Choosing a safe speed, keeping a space cushion, adjusting for weather and traffic, and understanding stopping distance.
What to do when things go wrong — brake failure, tire blowouts, skids, and stalling on railroad tracks.
How to pass the first time
- Read the official Alabama 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.
Alabama CDL — Air Brakes test — frequently asked questions
How many questions are on the Alabama CDL — Air Brakes test?
The Alabama 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 Alabama 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 Alabama 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 Alabama 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 Alabama 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 Alabama driver handbook, practice by topic to target weak areas, and finish with full-length tests until you consistently pass.
Other Alabama practice tests
Take the CDL — Air Brakes test in another state
Every Alabama question is written from the official Alabama 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
Alabama CDL — Air Brakes questions with answers
20 sample questions from the Alabama CDL — Air Brakes question bank — each with the correct answer, a short explanation, and the section of the official handbook it comes from.
1. In an s-cam drum brake, what happens when air enters the brake chamber?
Correct answer: The push rod moves the slack adjuster and the s-cam forces the shoes against the drum
Air pressure pushes the rod out, which moves the slack adjuster and twists the camshaft; the s-cam then turns and forces the brake shoes apart against the inside of the drum.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.8 Foundation Brakes
2. On newer vehicles, how do you apply the parking (spring) brakes?
Correct answer: Pull out the yellow, diamond-shaped control knob
Newer air brake vehicles use a yellow, diamond-shaped push-pull knob; you pull the knob out to set the parking brakes and push it in to release them.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.15 Parking Brake Controls
3. If the low air pressure warning comes on while driving, why is it safer to stop while air still remains in the tanks?
Correct answer: You can still use the foot brakes for a controlled stop while air lasts
Controlled braking is only possible while enough air is left in the tanks; if you wait, the spring brakes may come on suddenly, and a heavily loaded vehicle takes a long distance to stop because they do not work on all axles.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.7 Low Air Pressure
4. On a dual air system, the low air warning should come on before pressure drops below 55 psi in which system?
Correct answer: Either system
The warning light and buzzer should activate before pressure falls under 55 psi in either the primary or the secondary system, and if that happens while driving you should stop and park safely.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.2 Dual Air Brake Systems
5. If the air compressor has its own separate oil supply, what should the driver do before a trip?
Correct answer: Check the level of that oil supply
A compressor that carries its own oil, rather than sharing the engine's oil, needs its oil level checked before driving so it stays properly lubricated.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.1 Air Compressor
6. On a tractor or straight truck, spring brakes will come fully on when air pressure drops into roughly what range?
Correct answer: About 20 to 45 psi
When air pressure falls to about 20 to 45 psi (often between 20 and 30), the springs are no longer held back and the spring brakes apply fully.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.14 Spring Brakes
7. When the low air pressure warning first comes on, what is the safest thing to do?
Correct answer: Bring the vehicle to a safe stop right away
You should bring the vehicle to a safe stop right away while you still have enough air to control the brakes, rather than waiting for the spring brakes to apply on their own.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.14 Spring Brakes
8. Before driving a vehicle with a dual air system, how much pressure should you let build up in both the primary and secondary systems?
Correct answer: A minimum of 100 psi in both systems
You should allow the compressor time to build a minimum of 100 psi in both the primary and secondary systems before you drive.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.2 Dual Air Brake Systems
9. During ordinary driving, which system engages and releases the brakes as you work the pedal?
Correct answer: The service brake system
The service brake system is the one at work in everyday driving; it puts the brakes on and lets them off as you press and ease off the foot pedal.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - overview
10. With the engine off and a fully charged system, what is the maximum acceptable air loss rate for a single vehicle?
Correct answer: Less than 2 psi in one minute
During the leakage test on a fully charged system with the engine off, a single vehicle should lose less than 2 psi in one minute (combination vehicles are allowed less than 3 psi per minute).
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.3.3 Final Air Brake Check
11. When using the 'controlled braking' method in an emergency, how hard should you press the brakes?
Correct answer: As hard as you can without locking the wheels
Controlled braking means applying the brakes as hard as you can without locking the wheels while keeping steering movements small; if the wheels lock or you must steer more, release and then reapply.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.3 Emergency Stops
12. For a vehicle with air brakes, total stopping distance is made up of which four parts?
Correct answer: Perception, reaction, brake lag, and braking distance
Because of brake lag, total stopping distance for air brakes adds four parts together: perception distance, reaction distance, brake lag distance, and braking distance.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.4 Stopping Distance
13. Which type of foundation brake is the most common on vehicles with air brakes?
Correct answer: The s-cam drum brake
Although wedge and disc designs exist, the s-cam drum brake is the most widely used foundation brake found at the wheels.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.8 Foundation Brakes
14. Why should you avoid pressing the brake pedal while the spring brakes are already applied?
Correct answer: The combined forces can damage the brakes
Doing so combines the force of the springs with the air pressure, which can damage the brakes, so it is better never to press the pedal when the spring brakes are on.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.15 Parking Brake Controls
15. A visible low air pressure warning must come on before tank pressure drops below what level?
Correct answer: Below 55 psi
Vehicles with air brakes are required to give a warning the driver can see before pressure in the tanks falls under 55 psi.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.11 Low Air Pressure Warning
16. If one half of a dual air system loses most of its pressure, what should you expect?
Correct answer: It will take longer to stop the vehicle
With one system very low, either the front or the rear brakes will not be fully operating, so it will take you longer to stop; bring the vehicle to a safe stop and have it repaired.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.2 Dual Air Brake Systems
17. Why can the brakes still be used several times even if the compressor quits working?
Correct answer: The tanks hold a reserve of compressed air
The storage tanks keep a reserve of compressed air, so there is enough on hand to apply the brakes a number of times even after the compressor stops.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.3 Air Storage Tanks
18. Why is it important to drain the air tanks completely?
Correct answer: To remove water and oil that could cause brake failure
Compressed air carries some water and compressor oil that settle at the bottom of the tank; the water can freeze in cold weather and cause the brakes to fail, so the tanks must be drained.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.4 Air Tank Drains
19. What is the main cause of brake fade?
Correct answer: Excessive heat from overusing the service brakes
Brake fade comes from excessive heat that builds up when the service brakes are used too much instead of relying on the braking effect of the engine; the heat changes the lining and expands the drums, reducing braking.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.5 Brake Fading or Failure
20. What best describes a dual air brake system?
Correct answer: Two separate air systems sharing one set of controls
A dual air brake system is two separate air brake systems that share a single set of brake controls, with each system having its own tanks, hoses, and lines.
Source: Alabama Commercial Driver License Manual, Section 5 (Air Brakes) - 5.2 Dual Air Brake Systems