Free Missouri DMV CDL — Air Brakes Practice Test 2026
The Missouri CDL — Air Brakes knowledge test has 25 questions, and you need 20 correct (80%) to pass. Practice free with questions written from the official Missouri 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 Missouri written test
The Missouri CDL — Air Brakes knowledge test has 25 questions; you need 20 correct (80%) to pass.
Based on the official Missouri 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 Missouri 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.
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 Missouri 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.
Missouri CDL — Air Brakes test — frequently asked questions
How many questions are on the Missouri CDL — Air Brakes test?
The Missouri 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 Missouri 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 Missouri 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 Missouri 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 Missouri 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 Missouri driver handbook, practice by topic to target weak areas, and finish with full-length tests until you consistently pass.
Other Missouri practice tests
Take the CDL — Air Brakes test in another state
Every Missouri question is written from the official Missouri 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
Missouri CDL — Air Brakes questions with answers
20 sample questions from the Missouri 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. What is the purpose of an alcohol evaporator in some air brake systems?
Correct answer: It lowers the chance of ice forming in the valves during cold weather
An alcohol evaporator adds alcohol to the air to reduce the risk of ice building up in the valves and other parts in cold weather, since ice can stop the brakes from working.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.5 Alcohol Evaporator
2. When you pull on an s-cam slack adjuster by hand, about how much movement suggests it needs adjustment?
Correct answer: More than roughly one inch where the push rod attaches
If a slack adjuster shifts more than about an inch at the push-rod connection, it likely needs adjustment; poorly adjusted brakes are the fault inspectors most often write up at the roadside.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.3.2 Walk-around Inspection
3. On a long, steep downgrade, the service brakes should be used how?
Correct answer: As a supplement to the engine's braking, after you are in a low gear
On a downgrade the brakes only back up the engine's braking effect; select the proper low gear first, then brake in short applications.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.6 Proper Braking Technique
4. How does the air brake system turn on your brake lights for drivers behind you?
Correct answer: An electric switch operated by air pressure lights the lamps when you brake
An air-pressure-operated electric switch turns the brake lights on whenever the air brakes are applied.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.12 Stop Light Switch
5. At about what pressure do a tractor or straight truck's spring brakes fully engage on their own?
Correct answer: 20 to 45 psi
Spring brakes on tractors and straight trucks come fully on when air pressure falls into the 20-to-45 psi range.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.14 Spring Brakes
6. How does ABS relate to your normal braking ability?
Correct answer: It is an add-on that leaves normal braking unchanged and works only near lockup
ABS adds to your regular brakes without reducing or boosting them, activating only when the wheels are about to lock; it will not necessarily shorten stopping distance.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.16 Antilock Braking Systems
7. How do you check that the parking brake will hold?
Correct answer: Set the parking brake and gently try to pull forward in a low gear
With the vehicle stopped, set the parking brake and, in a low gear, tug lightly forward to confirm it holds the rig.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.3.3 Final Air Brake Check
8. Why do air brakes have 'brake lag' that hydraulic brakes do not?
Correct answer: It takes a moment for the air to travel through the lines to the brakes
Unlike hydraulic brakes that act instantly, air brakes need a fraction of a second for air to move through the lines, adding brake-lag distance.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.4 Stopping Distance
9. What is a 'wig wag' device?
Correct answer: A mechanical arm that drops into the driver's view when pressure sinks below 55 psi
The wig wag is a mechanical arm that falls into view to warn of low air; an automatic one lifts back out of view once pressure climbs above 55 psi.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.11 Low Air Pressure Warning
10. When the low air warning light and buzzer first come on, what should you do?
Correct answer: Stop the vehicle safely without delay, while you still have control over the brakes
Do not wait for the spring brakes to trip automatically; stop safely as soon as the warning appears, while you still have braking control.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.14 Spring Brakes
11. What slows the vehicle when the shoes press against the drum, and what byproduct results?
Correct answer: Friction slows the vehicle while generating heat
Pressing the linings against the drum creates friction that slows the vehicle and also produces heat; too much heat can make the brakes quit.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.8 Foundation Brakes
12. How large a crack in a brake drum is unacceptable during inspection?
Correct answer: A crack running more than half the width of the friction area
A drum crack that extends beyond half of the friction surface's width is unacceptable; linings must also stay secure, free of oil or grease, and not worn too thin.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.3.2 Walk-around Inspection
13. The 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 stopping distance combines perception distance, reaction distance, brake-lag distance, and braking distance.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.4 Stopping Distance
14. The low air pressure warning must come on before tank pressure falls below what level?
Correct answer: 55 psi
A visible low-pressure warning is required and must activate before pressure in the tanks drops under 55 psi.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.11 Low Air Pressure Warning
15. What is the safest response if the low-pressure warning lights up while you are driving?
Correct answer: Pull over and park safely as soon as you can, while air still remains
A low-air warning may mean a leak; stop and park safely right away, because controlled braking is only possible while enough air is left in the tanks.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.7 Low Air Pressure
16. An air brake arrangement is really a combination of three braking functions. Which set correctly names them?
Correct answer: The service, parking, and emergency brakes
Air brakes bring together three systems: the service brakes for everyday stops, the parking brakes for holding the rig, and the emergency brakes that can halt it if the system fails.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - Section overview
17. How often must you empty air tanks fitted with manually operated drain valves?
Correct answer: At the end of each day you drive
Tanks with manual drains have to be emptied yourself at the close of every driving day.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.4 Air Tank Drains
18. At 55 mph on dry pavement, about how much distance does air brake lag add?
Correct answer: About 32 feet
Brake lag at 55 mph on dry pavement adds roughly 32 feet, contributing to a total stopping distance of over 450 feet.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.4.4 Stopping Distance
19. What is the benefit of a modulating control valve for the spring brakes?
Correct answer: It applies the spring brakes gradually, giving control if the service brakes quit
A modulating valve lets you bring the spring brakes on gradually, so you can still control the vehicle if the service brakes fail.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.15 Parking Brake Controls
20. Current guidance says front-wheel braking is:
Correct answer: Helpful in all conditions, with skids unlikely even on ice
Front-wheel braking improves stopping power in every condition, and tests show that front-wheel skids from braking are unlikely even on ice.
Source: Missouri Commercial Driver License Manual, Section 5 (Air Brakes) - 5.1.13 Front Brake Limiting Valve