CDL Prep HeroRead the explanation after every answer. No account needed.
On an S-cam brake, air enters the brake chamber when you press the pedal. What happens?
What the Air Brakes test covers
Questions come from Section 5 of your state's CDL manual. You'll need to know the parts of the system (compressor, governor, tanks, drains, gauges, spring brakes), how dual air systems work, how to inspect the brakes, and how to use them on the road.
Most states give about 25 questions, and you need 80% to pass.
The pressures you have to know
Do not memorize an unlabeled list of pressures. A knowledge-test manual, a modernized skills-test checklist, and the vehicle manufacturer may describe different checks or limits. Use the current manual for your state and the equipment you will test in.
For each pressure or leakage limit, identify the test conditions: engine running or off, brakes applied or released, and single vehicle or combination. Ask your instructor to demonstrate the correct procedure before practicing it on a vehicle.
How to study for it
Learn the parts first, then the numbers, then the inspection sequence. The inspection steps make more sense once you know what each part does, and the numbers stick better when you know what they're checking.
Common questions
Do I need the Air Brakes test?
Only if you'll drive a vehicle with air brakes. If you don't pass it, your CDL gets an air brake restriction, and you can't drive a CMV with air brakes.
How many questions are on the CDL Air Brakes test?
Most states give 25 questions, and you need 80% to pass.
What's the hardest part of the Air Brakes test?
The pressure checks can be easy to confuse. Keep governor operation, the low-air warning, spring-brake activation, and the different leakage checks separate. Match each check to your current state manual and vehicle.
Questions and explanations
Prefer to read? Review the same 25 practice questions here. These are original study questions, not official exam questions. Our practice questions include explanations to help you return to the relevant official manual material.
On an S-cam brake, air enters the brake chamber when you press the pedal. What happens?
Chamber pressure pushes a rod that moves the slack adjuster and brake.
That's how an S-cam service chamber works. Air pressure pushes the rod, and the rod moves the slack adjuster and brake.
A vehicle has manual air-tank drains. When should you drain the tanks?
At the end of each driving day, and drain them all the way.
With manual drains, you do the draining. Drain the tanks fully at the end of each driving day.
Which inspection plan correctly matches how the air compressor is driven?
Check belt condition and tightness on a belt-driven compressor. Don't assume a gear-driven one has a belt.
The compressor may be gear-driven or V-belt driven. The condition-and-tightness check applies to the belt-driven kind.
One circuit in a dual air-brake system gets very low while you're driving. What should you do?
Stop safely and have the system fixed.
Brakes on the low circuit may not work fully. Stopping can take longer, so driving on is unsafe.
You're done with the leakage check before leaving. The vehicle is secured and the test was set up right. The air loss is above the limit for that vehicle. What should you decide?
Have the vehicle checked for leaks and fixed before you drive.
Leakage over the limit is a no-go defect. You could lose braking while moving.
During a planned normal stop, how should you use the brake pedal?
Adjust pedal pressure so the vehicle comes to a smooth, safe stop.
A normal stop uses adjusted brake pressure to get a smooth, safe result.
What does the air compressor governor control?
Whether the compressor pumps air into the air tanks.
The governor stops the compressor pumping at cut-out and lets it pump again at cut-in.
Which description matches an automatic air-tank drain?
It lets out water and oil on its own, and the tank may also have a manual drain.
Automatic drains let out the collected water and oil on their own. Some of those tanks also have a manual drain.
A brake lining is dry, with no oil or grease on it, but it's loose. What does the walk-around check tell you?
The lining fails, because being loose is its own problem.
A lining must not be loose. Being free of oil or grease doesn't cancel that separate rule.
Which description best fits a dual air-brake system?
Two separate systems run by one set of controls.
The two systems have their own tanks, hoses, and lines. They share the driver's brake controls.
You're making a normal stop in a manual-transmission vehicle. When should you push in the clutch?
When engine rpm is close to idle.
For a normal stop, keep the clutch engaged until engine rpm is close to idle.
A vehicle has an application-pressure gauge. What does it show?
How much air pressure you are applying to the brakes.
The application gauge shows the brake pressure you're calling for with the pedal.
Which part holds the reserve of compressed air for the air brakes?
The air tank, or reservoir.
The tanks hold compressed air. They give you a reserve when the compressor isn't pumping.
Some automatic air-tank drains have an electric heater. What does the heater do?
It helps keep the automatic drain from freezing in cold weather.
The heater helps the automatic drain work by keeping it from freezing in cold weather.
You're checking an air hose at a brake chamber. What visible problems are you looking for?
Cuts, and wear caused by rubbing.
The walk-around hose check names both cuts and rubbing wear.
What should you know about which axles each circuit of a dual system serves?
The exact axle split depends on the vehicle.
A front-and-rear split is common, but the exact layout depends on the vehicle.
You're done with the buildup check before leaving. The vehicle is secured. Even after allowing extra time for larger tanks, the result fails the rule for that vehicle or its maker. What should you decide?
Have the buildup problem fixed before you drive.
A failed buildup check is a no-go defect. Pressure could fall too low while moving.
Which pattern most directly raises the risk of brake heat and fade during long braking?
Using the service brakes hard or for a long time without help from engine braking.
Friction heat depends on how hard and how long the brakes work. Overuse can cause fade.
Which pair correctly describes the governor's cut-out and cut-in?
Cut-out stops pumping into the tanks. Cut-in lets pumping start again.
Those are the governor's two control states.
It's cold weather and the vehicle has an alcohol evaporator. What should you do each day?
Check its container and fill it as needed.
In cold weather, check the evaporator container every day and fill it as needed.
All the brake hardware is present, but one part is visibly broken. How does that stack up against the walk-around check?
It fails, because parts must be present and not broken or missing.
Being present isn't enough. Brake parts must also not be broken or missing.
A low-air warning comes on while you're moving. What's the safe response?
Stop and park safely as soon as you can.
The warning means pressure in at least one circuit is too low. Make a safe stop right away.
What's the risk to the whole system when some brakes are out of adjustment?
The other brakes have to do more work and may overheat and fade.
Brakes that stop sharing the load push more work and heat onto the brakes still working right.
A vehicle has a dual air system. Two needles show on one supply gauge. What can they mean?
Separate supply-pressure readings for the two air systems.
A dual system may use two gauges or two needles on one supply gauge, one for each system.
A vehicle has an alcohol evaporator and manual tank drains. Which cold-weather plan keeps both jobs straight?
Check and fill the evaporator as needed each day, and still drain the tanks daily.
The evaporator cuts the risk of ice in air-system parts. Manual tanks still need draining every day.
Study sources and limitations
Editorial attribution: Evergreen Comply, LLC Editorial Staff. Our practice questions include explanations to help you return to the relevant official manual material.
Use your current state manual and a qualified instructor for licensing requirements and hands-on procedures. This practice set does not replace required training or certify readiness to drive.
- Texas DPS: CDL application resources and official manual
- New York DMV: CDL resources and manual
- New York CDL manual: Air Brakes (PDF)
How we handle sources and corrections · Report a question or explanation
Source links checked September 25, 2026.
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