CDL Air Brakes Practice Test 2026 — Free
Free CDL air brakes practice test for 2026: 20 questions with answers on the air brake system, leak tests and warnings — fail it and your CDL is restricted.
What's on the Air Brakes CDL test
Answering from memory beats re-reading — retrieval practice roughly doubles what you remember a week later. These free questions are modeled on the official Air Brakes CDL knowledge test.
- Official test
- 25
- Practice questions here
- 20
- To pass
- 20/25 correct (80%)
- Updated
- September 2026
- 100% free
CDL air brakes test at a glance
- What it is
- A knowledge test, not an endorsement — skip or fail it and your CDL carries an air brake restriction (code L)
- Test type
- Knowledge test; your skills test must also be taken in an air-braked vehicle to avoid the restriction
- Who needs it
- Anyone who will drive a CMV with air brakes — nearly every Class A tractor and most Class B trucks and buses
- Where to study
- The “Air Brakes” section of your state CDL manual (Section 5 in the model manual)
- Official test
- 25 questions under the AAMVA model most states use — your state may differ
- Pass mark
- 80% — 20 of 25 correct (federal, 49 CFR 383.135)
- This practice set
- 20 questions with explanations, free, no sign-up
Show all 20 practice questions with answers
If your vehicle doesn't have automatic tank drains, you should drain the oil and water from the bottom of the air tanks ___.
- Before starting the engine each morning.
- At the end of each day of driving.
- Every month.
Correct answer: At the end of each day of driving.
Drain the oil and water from your truck's air tanks at the end of each day of driving if they don't have auto drains. This keeps the brakes working right.
What does the air compressor governor control?
- The amount of fuel the engine uses
- When air is pumped into the air tanks.
- The temperature of the engine.
Correct answer: When air is pumped into the air tanks.
The air compressor governor decides when to fill up the air tanks used for brakes and other parts. It doesn't control the engine's fuel use or temperature.
If you pull hard on a slack adjuster and it moves more than ___, it needs to be adjusted.
- 1 inch
- 5/8 inch
- 3 inches
Correct answer: 1 inch
If the slack adjuster moves more than 1 inch when you pull on it, it's too loose and needs adjusting. This is important for the brakes to work right.
If the air pressure doesn't go up to the needed level fast enough, then:
- The headlights could become dimmer.
- You might need to adjust the mirrors more frequently.
- Air pressure may drop to a low point while driving, requiring an emergency stop.
Correct answer: Air pressure may drop to a low point while driving, requiring an emergency stop.
If the air pressure in your truck doesn't build up fast enough, it can get too low while you're driving. This means you might have to suddenly stop your truck to avoid problems.
Parking or emergency brakes must be kept in position by something that does not leak, like:
- Spring pressure.
- Magnetic force
- Electric current
Correct answer: Spring pressure.
The right answer is spring pressure. This is because parking or emergency brakes in vehicles need a reliable way to stay active. Springs can do this without the risk of losing power, unlike systems that depend on magnets or electricity, which can fail if there's a power loss or leak.
Excessive heat caused by overusing your brakes can also cause ___.
- Tire pressure to decrease.
- Brake to fade or fail.
- Engine oil to overheat.
Correct answer: Brake to fade or fail.
Using your brakes too much can make them too hot. When they get too hot, they might not work right or could stop working altogether.
At what psi does the air compressor stop pumping air?
- 135
- 125
- 140
Correct answer: 125
The air compressor stops pumping air at 125 psi because this is the standard pressure level needed to maintain the air brake system's functionality without overloading it. This ensures that the system has sufficient air pressure to operate safely while preventing potential damage or inefficiencies from excessive pressure.
What brake system applies and releases the brakes when the driver steps on the brake pedal?
- Manual
- Service
- Automatic
Correct answer: Service
The correct answer is "Service." This is the brake system in a vehicle that works when the driver pushes the brake pedal to slow down or stop the truck. It's the main system used for everyday driving.
The safety valve lowers pressure when it reaches ___ psi.
- 150
- 100
- 50
Correct answer: 150
The safety valve helps keep the air pressure in the brakes from getting too high. It does this by letting some air out when the pressure hits 150 psi. This makes sure the brakes work right and stay safe.
What happens if your vehicle's brake drums overheat?
- They expand.
- They crack and leak fluid.
- They glow bright red
Correct answer: They expand.
When your brake drums get very hot, they expand. This means they get bigger because of the heat.
Why is it important to remove water from the compressed air tanks?
- Water can freeze in cold weather and cause brake failure.
- Water is removed because its presence might cause the tank to change color, which is thought to be a sign of wear.
- Water is removed from the compressed air tanks to make the system lighter and improve fuel efficiency.
Correct answer: Water can freeze in cold weather and cause brake failure.
Draining water from air tanks is important because in cold weather, it can freeze and stop your brakes from working right.
When going down a steep hill and reaching your safe speed of 40 mph, you would use the brake until your speed goes down to _____ mph.
- 35
- 30
- 25
Correct answer: 35
When driving down a steep hill and you're at your safe speed of 40 mph, hit the brakes until you slow down to 35 mph. Then, let go of the brakes to avoid overusing them.
When you use the service brakes too much, they can get too hot and this can cause ___.
- Decreased fuel efficiency
- Reduced tire pressure
- Expansion of the brake drums.
Correct answer: Expansion of the brake drums.
When you use the brakes too much, they can get really hot. This heat can make the brake drums (part of the brakes) get bigger. This doesn't help the car save gas or change the air in the tires; it just makes the brakes work less effectively.
Your truck is equipped with a dual air brake system and minimum-size air tanks. How long should it take for the air pressure to go up from 85 to 100 psi?
- 30
- 45
- 60
Correct answer: 45
When your truck's air brake system is in good shape and the tanks are the smallest allowed, the air pressure should go from 85 to 100 psi in 45 seconds or less. This shows your brakes are working right and can safely stop the truck.
What does the term "Wig Wag" mean?
- A mechanical arm that signals low air pressure
- A lever that controls the speed of the engine.
- A light that flashes when the engine is overheating
Correct answer: A mechanical arm that signals low air pressure
The term "Wig Wag" refers to a safety device in air brake systems that uses a mechanical arm to alert the driver when air pressure drops to a dangerously low level. This visual warning helps ensure the driver takes immediate action to prevent brake failure, maintaining safety on the road.
What statement about spring brake braking power is correct?
- It depends on the service brakes being in adjustment.
- It is most effective when the vehicle is fully loaded.
- It decreases as the vehicle's speed increases.
Correct answer: It depends on the service brakes being in adjustment.
The spring brakes on a vehicle work properly only if the regular brakes (service brakes) are set up right. If the service brakes are not adjusted correctly, the spring brakes won't be able to do their job well. So, the power of the spring brakes relies on the service brakes being properly adjusted.
How do you normally engage the parking brakes?
- Apply brakes by pulling the parking brake control knob out.
- Push the gas pedal down.
- Shift into neutral gear.
Correct answer: Apply brakes by pulling the parking brake control knob out.
Engaging the parking brake is all about ensuring your car stays put once parked. By pulling out the control knob, you directly activate the brake system that keeps the vehicle stationary.
When the air pressure falls below ___ psi, the straight truck spring brakes will come fully on.
- 20
- 60
- 100
Correct answer: 20
When driving a straight truck, if the air pressure gets too low, special brakes called spring brakes kick in to stop you. This happens when the air pressure drops below 20 psi (pounds per square inch). It's a safety feature to prevent accidents when there's not enough air pressure for the regular brakes.
What is the purpose of a brake light switch?
- It turns on the brake lights when you put on the air brakes.
- It adjusts the brightness of your headlights.
- It activates the turn signals when you apply the brakes
Correct answer: It turns on the brake lights when you put on the air brakes.
A stop light switch makes the brake lights turn on when you use the air brakes. This lets other drivers know you're slowing down or stopping.
Which of the following types of foundation brakes are most commonly found on heavy vehicles?
- S-cam drum.
- Hydraulic disc
- Air pad
Correct answer: S-cam drum.
The S-cam drum brake is the most common brake on big vehicles. It uses a special cam to push brake pads against a drum to stop the vehicle. This system is reliable and widely used in trucks and buses.
What the CDL air brakes test is — and who needs it
Air brakes is not an endorsement: no letter is added to your license for passing it. It works the other way around. If you fail the air brake component of the knowledge test, or take your skills test in a vehicle that is not equipped with air brakes, the state must mark your learner’s permit or CDL with a restriction that bars you from operating any commercial motor vehicle equipped with any type of air brakes.
That restriction is printed as code L — no air-brake-equipped CMV. A second, narrower restriction, code Z, applies when you take the skills test in a vehicle with air-over-hydraulic brakes: you may drive those, but not a vehicle braked fully on the air principle. For the test and the restriction, “air brakes” means any braking system operating fully or partially on the air brake principle.
Because virtually every tractor-trailer and most straight trucks and buses over 26,001 pounds use air brakes, this test is effectively mandatory for a working Class A or Class B CDL. Removing the restriction later does not require a complete retest: the state may give a modified skills test, and you must perform the air brake pre-trip inspection and pass the air brake knowledge test.
49 CFR 383.95(a)–(b), 383.135(a)(2), 383.135(b)(3)–(4) and (b)(7), 383.153(a)(10)
Drivers who need to pass the air brakes test
Take the air brakes test with your permit tests if any of these describe you:
- Every Class A applicant — tractor-trailers are air-braked, and the federal combination vehicles knowledge areas include air brakes.
- Class B drivers of dump trucks, box trucks, refuse trucks, transit buses and motorcoaches with air brakes.
- Drivers whose employer’s fleet uses air brakes even if the truck you tested in did not — the restriction follows the license, not the job.
- Drivers already carrying an L or Z restriction who want it removed.
49 CFR 383.111(c)(3), 383.95(a), 383.135(b)(7)
What is on the air brakes knowledge test
Federal rule names seven knowledge areas every air brake test must cover. Expect questions on:
- Air brake system nomenclature — compressor, governor, tanks, valves, chambers, slack adjusters.
- The dangers of a contaminated air supply: dirt, moisture and oil in the system.
- What happens when the air lines between the power unit and the trailer are severed or disconnected.
- What a low air pressure reading means and what to do about it.
- Pre-trip inspection procedures, including automatic fail-safe devices, system monitoring devices and low-pressure warning alarms.
- En-route and post-trip inspections: detecting defects, air-loss (leak-down) tests with and without the engine running, and the pressure levels at which the low-pressure warning and the tractor protection valve should activate.
- Operating practices: proper braking techniques, antilock brakes, emergency stops and the parking brake.
49 CFR 383.111(b)
How to pass the air brakes test and avoid the restriction
Take the air brakes knowledge test with your permit tests
The air brake component is part of the knowledge testing that leads to a commercial learner’s permit. Skip it or fail it and the permit itself is issued with the air brake restriction. For a Class A, plan on three knowledge tests: general knowledge, combination vehicles and air brakes.
CDL Class A practice test49 CFR 383.135(a)(2), 383.135(a)(3)
Study the air brakes section of your state’s CDL manual
In the AAMVA model manual most states publish, air brakes is Section 5: the parts of the system, the dual air brake system, inspecting air brakes (the leak-down, warning and tractor-protection-valve checks) and using air brakes. The seven federal knowledge areas above are what the questions are built from; drill the practice set on this page until you are consistently above 80%.
Pass the knowledge test at your state licensing agency
You need at least 80%, the federal pass mark for every CDL knowledge test. Under the AAMVA model the air brakes test has 25 questions — 20 correct to pass — though your state can use a different count. Testing fees, appointment rules and retake waits vary by state; the state pages below name the agency that issues your license.
Find your state’s air brakes practice test and agency49 CFR 383.135(a)(1)
Take your skills test in a vehicle with air brakes
Passing the knowledge test is only half of it. If you perform the skills test in a vehicle without air brakes, the state must still place the air brake restriction on your CDL; test in a vehicle with air-over-hydraulic brakes and you get the narrower full-air-brake restriction instead. Test in a fully air-braked truck to keep your license clean.
49 CFR 383.95(a)(1)–(b), 383.135(b)(3)–(4)
Already restricted? Remove the L or Z with a modified skills test
You do not have to retake the complete skills test. The state may administer a modified skills test showing you can safely operate the vehicle’s full air brakes, and you must perform the air brake pre-trip inspection and pass the air brake knowledge test.
49 CFR 383.135(b)(7)
CDL air brakes test — questions and answers
Is air brakes an endorsement?
No. It is a knowledge test that removes a restriction rather than adding an endorsement. Fail the air brake portion of the knowledge test — or take the skills test in a vehicle without air brakes — and your permit or CDL must show a restriction against operating any CMV equipped with air brakes.
49 CFR 383.95(a), 383.135(a)(2)
What do the L and Z restrictions mean?
L means no air-brake-equipped CMV: you failed or skipped the air brake knowledge test, or tested in a vehicle without air brakes. Z means no full-air-brake CMV: you took the skills test in a vehicle with air-over-hydraulic brakes, so you may drive those but not a vehicle whose brakes operate fully on the air principle.
49 CFR 383.153(a)(10), 383.95(a)–(b)
How many questions are on the air brakes test, and what score do I need?
Under the AAMVA model most states use, the air brakes test has 25 questions, and the federal pass mark of 80% means 20 correct. Your state can use a different count — check its CDL manual. The practice set on this page plays 20 questions with explanations, so aim for at least 16 here before test day.
49 CFR 383.135(a)(1)
Can I take the skills test in a hydraulic-brake truck and add air brakes later?
Yes, but your CDL will carry the air brake restriction until you remove it. Removal does not require a complete retest: the state may give a modified skills test, and you must perform the air brake pre-trip inspection and pass the air brake knowledge test.
49 CFR 383.135(b)(3), 383.135(b)(7)
Do I need the air brakes test for a Class B CDL?
Only if the vehicles you will drive have air brakes — but most Class B trucks and buses rated 26,001 pounds or more do. Without the test your license is limited to CMVs with hydraulic brakes, which rules out most Class B jobs. The Class B hub lists the knowledge tests that class needs.
CDL Class B practice test49 CFR 383.95(a)
Is the air brakes test the same in every state?
The seven knowledge areas are federal, so the subject matter is the same everywhere; question counts, fees and scheduling rules are set by each state. Pick your state below for the agency that administers the test.
49 CFR 383.111(b)
Official sources
- 49 CFR 383.111 — Required knowledge (air brakes, paragraph (b))
- 49 CFR 383.95 — Restrictions
- 49 CFR 383.135 — Passing knowledge and skills tests
- FMCSA — Commercial Driver’s License
Fees, the licensing agency and appointment rules vary by state — pick your state below for the agency that issues your license.
Federal rules on this page last reviewed against the eCFR on .
Practice the Air Brakes test by state
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