Free Mississippi DMV CDL — Air Brakes Practice Test 2026
The Mississippi CDL — Air Brakes knowledge test has ~25 questions, and you need ~20 correct (80%) to pass. Practice free with questions written from the official Mississippi 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 Mississippi written test
The Mississippi CDL — Air Brakes knowledge test has ~25 questions; you need ~20 correct (80%) to pass.
Based on the official Mississippi 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 Mississippi practice questions.
How to pass the first time
- Read the official Mississippi 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.
Mississippi CDL — Air Brakes test — frequently asked questions
How many questions are on the Mississippi CDL — Air Brakes test?
The Mississippi 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 Mississippi 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 Mississippi 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 Mississippi 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 Mississippi 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 Mississippi driver handbook, practice by topic to target weak areas, and finish with full-length tests until you consistently pass.
Other Mississippi practice tests
Take the CDL — Air Brakes test in another state
Every Mississippi question is written from the official Mississippi 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
Mississippi CDL — Air Brakes questions with answers
20 sample questions from the Mississippi 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. You are warming up a large bus, and its low pressure warning signals at about 80 psi, well above 55. What does the manual say about this?
Correct answer: Warnings that signal at 80 to 85 psi are common on large buses
The 55 psi figure is the latest point a warning may come on. Large buses commonly have warning devices set higher, at 80 to 85 psi.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Low Air Pressure Warning)
2. If the low air warning signal does not work, what could happen in a single-circuit air brake system?
Correct answer: You could lose air pressure without knowing it and get sudden emergency braking
Without a working warning you may not notice falling pressure. In a single-circuit system that can mean sudden emergency braking; in a dual system it means a longer stopping distance.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Final Air Brake Check)
3. A parked truck slowly loses all its air overnight through a small leak. Why does its parking brake keep holding?
Correct answer: Its spring brakes are applied by mechanical spring force, which does not depend on air
Parking and emergency brakes have to be held on mechanically because air can leak away. Spring brakes do this: losing air actually lets the springs apply the brakes.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Spring Brakes)
4. You are driving an air-braked truck at 55 mph on dry pavement when you spot a hazard. With good traction and brakes, about how much road will you cover from seeing it to stopping?
Correct answer: More than 450 feet
For an average driver at 55 mph in good conditions, total stopping distance with air brakes tops 450 feet. Brake lag alone adds roughly 32 feet of that.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Stopping Distance)
5. In an s-cam brake, air entering the brake chamber pushes the rod out. Which part does the rod move next, twisting the brake camshaft?
Correct answer: The slack adjuster
The push rod moves the slack adjuster, which twists the camshaft and turns the S-shaped cam. The cam then spreads the shoes against the inside of the drum.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Foundation Brakes)
6. In stop-and-go traffic you keep pressing and releasing the brake pedal far more than you need to. What could this lead to?
Correct answer: Air leaving the system faster than the compressor can replace it, until the brakes no longer work
Each time you release the pedal, some compressed air is let out and the compressor has to make it up. Needless fanning can drain pressure faster than it is rebuilt, and with too little pressure the brakes stop working.
Source: Mississippi CDL Manual — Section 5: Air Brakes (The Brake Pedal)
7. Your brakes are badly out of adjustment. How does that affect the spring brakes?
Correct answer: Both the regular brakes and the emergency/parking brakes will be weakened
How well spring brakes hold depends on the brakes being properly adjusted. Poor adjustment weakens the regular brakes and the emergency/parking brakes alike.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Spring Brakes)
8. Total stopping distance for a vehicle with air brakes is made up of four distances. Which of these is NOT one of them?
Correct answer: Coasting distance
The four parts are perception, reaction, brake lag, and braking distance. Brake lag is the extra piece air brakes add because air needs time to reach the brakes.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Stopping Distance)
9. A brake fitted with an automatic slack adjuster shows a pushrod stroke past the legal limit. What does this generally indicate?
Correct answer: A mechanical problem in the adjuster or related brake parts, or a poor installation
An automatic adjuster that lets the stroke exceed the limit signals a fault in the adjuster, in the foundation brake parts, or in how it was installed. Adjusting it by hand hides the problem rather than fixing it.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Walk-around Inspection)
10. Your truck has a brake system failure. To bring it to a stop, the emergency brake system relies on parts of which other systems?
Correct answer: The service brake and parking brake systems
There is no fully separate emergency brake. When something fails, the emergency system borrows components from the service brakes and the parking brakes to stop the vehicle.
Source: Mississippi CDL Manual — Section 5: Air Brakes
11. You park after driving through slush, the brakes are soaked, and the temperature is below freezing. Why might setting the parking brakes be a mistake?
Correct answer: They could freeze so the vehicle cannot move
Very wet brakes in freezing weather can freeze in place. Hold the vehicle with wheel chocks on level ground instead, and dry the brakes by braking lightly in a low gear.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Parking Brakes)
12. A new driver says his truck has ABS, so he can follow closer because he will always stop shorter. What does the manual say?
Correct answer: ABS does not necessarily shorten stopping distance, but it helps keep the vehicle under control in hard braking
ABS neither adds to nor takes away from normal braking ability. It steps in only when wheels are about to lock, which helps control but does not promise a shorter stop.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Antilock Braking Systems (ABS))
13. You watch your air gauge climb to about 125 psi and then hold steady. What is the most likely reason it stopped rising?
Correct answer: The governor reached its cut-out point and stopped the compressor from pumping
Once the tanks reach the cut-out level of roughly 125 psi, the governor halts the compressor's pumping. A reading that levels off there is normal.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Air Compressor Governor)
14. If your service brakes fail, how can a modulating control valve help you stop?
Correct answer: It lets you apply the spring brakes gradually, harder the farther you move the lever
A modulating valve is a spring-loaded dash handle that gives you a feel for braking. The more you move it, the harder the spring brakes apply, so you can control them when the service brakes are gone.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Parking Brake Controls)
15. Which sequence matches the manual's normal stop in an air-braked truck with a manual transmission?
Correct answer: Press the pedal and ease off as needed, clutch in once rpm nears idle, then pick a starting gear when stopped
Control the pedal pressure for a smooth stop and keep the clutch out until engine speed falls near idle. Once stopped, choose a starting gear.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Normal Stops)
16. On a long downgrade in the right low gear, your 'safe' speed is 50 mph. You just braked down to about 45 mph and let off. When do you brake again?
Correct answer: When your speed climbs back up to 50 mph
The technique is a cycle: brake firmly at your safe speed until you are about 5 mph under it, release, and brake again once you are back at the safe speed. Keep repeating until the grade ends.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Proper Braking Technique)
17. During the final air brake check, with the engine idling, how do you lower tank pressure to see whether the compressor cuts back in?
Correct answer: Step on and off the brake pedal
Each brake application uses some air. Stepping on and off the pedal brings the pressure down so you can confirm the compressor starts pumping again near the specified cut-in pressure.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Final Air Brake Check)
18. Your truck is nearly empty and the road is wet when a leak drains the air system and the spring brakes come on. What does the manual warn could happen?
Correct answer: The vehicle may skid out of control
When spring brakes apply on a lightly loaded vehicle or a slippery road, the vehicle can skid out of control. Stopping while you still have air for the foot brake is much safer.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Low Air Pressure)
19. When you pull out the parking brake knob on an air-braked truck, what actually makes the spring brakes apply?
Correct answer: Air is let out of the spring brakes, and the springs push the brakes on
While driving, air pressure holds the powerful springs back. The parking control releases that air, and the springs then apply the brakes.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Spring Brakes)
20. What is a downside of routinely adjusting automatic slack adjusters by hand?
Correct answer: It is likely to wear out the adjusters themselves early
Automatic adjusters should normally be adjusted by hand only during brake maintenance or installation. Doing it routinely tends to cause premature wear of the adjuster.
Source: Mississippi CDL Manual — Section 5: Air Brakes (Walk-around Inspection)