CDL Air Brakes Practice Test
This is a free 15-question sampler on the air brakes section of the CDL manual, written by the StudyPDF team. It covers the three braking systems, the compressor and governor, tanks and draining, the low air pressure warning, spring brakes, brake lag, braking on a long downgrade and the air brake portion of the pre-trip inspection, mixing multiple choice, select all that apply and fill in the blank. Answer all 15 questions, then check your score and read the explanation for every question.
Air brakes are really three separate braking systems. Which three?
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All 15 questions at a glance
Prefer to read before you play? Every question in the sampler is listed below. Answers and explanations stay collapsed until you open them.
1. Air brakes are really three separate braking systems. Which three?
- A. Service, hydraulic and parking
- B. Service, parking and emergency
- C. Primary, secondary and emergency
- D. Service, spring and modulating
Show answer and explanation
Answer: B. Service, parking and emergency
The service brake system applies and releases the brakes from the pedal during normal driving. The parking brake system holds the vehicle through the parking brake control. The emergency brake system is not a third set of hardware, it borrows parts of the other two so you still have a way to stop if the service system fails. Primary and secondary are the two halves of a dual air system, which is a different split entirely.
2. The governor decides when the air compressor pumps. On a typical vehicle, it stops and restarts the compressor at about which pressures?
- A. Cut out at about 60 psi, cut in at about 40 psi
- B. Cut out at about 100 psi, cut in at about 75 psi
- C. Cut out at about 125 psi, cut in at about 100 psi
- D. Cut out at about 150 psi, cut in at about 125 psi
Show answer and explanation
Answer: C. Cut out at about 125 psi, cut in at about 100 psi
The manual gives roughly 125 psi for cut out and roughly 100 psi for cut in, and the exact figures for your vehicle come from the manufacturer's specification. You test it by running the engine at a fast idle until the gauge stops climbing, then stepping on and off the brake to bleed pressure down until the needle starts rising again. Do not confuse 150 psi here, that is the safety valve, and it should never open in normal operation.
3. Complete the sentence.
A safety relief valve is fitted to the first tank the compressor pumps into, and it is usually set to open at _____.
Options for blank 1: 100 psi, 125 psi, 150 psi, 175 psi
Show answer and explanation
Answer: 150 psi
The safety valve protects the tanks and the rest of the system from too much pressure by venting, usually at about 150 psi. If it ever releases air, something upstream has failed, most often the governor, and a mechanic needs to look at it. Do not keep driving just because the brakes still seem to work.
4. The low air pressure warning light and buzzer come on while you are driving. What should you do?
- A. Stop and safely park the vehicle as soon as you can
- B. Pump the brake pedal to build the pressure back up
- C. Keep driving until the spring brakes come on by themselves
- D. Pull the parking brake knob out and continue at a lower speed
Show answer and explanation
Answer: A. Stop and safely park the vehicle as soon as you can
A warning you can see is required to come on before tank pressure falls below 60 psi, and it means air is leaving the system faster than the compressor can replace it. You still have controlled braking while air remains, so use it and get parked. Waiting for the spring brakes to grab is dangerous, because they do not act on every axle, a loaded vehicle takes a long distance to stop on them, and a light vehicle can skid out of control when they come on.
5. Which of the following are true about keeping water and ice out of an air brake system?
Select all that apply.
- A. Compressed air carries water and some compressor oil, which collect in the bottom of the tanks
- B. With manual drain valves, you drain the tanks at the end of each day of driving
- C. An alcohol evaporator puts alcohol into the system to reduce the risk of ice forming in the valves
- D. Filling the alcohol container replaces the need to drain the tanks
- E. Water in the tanks is harmless as long as the temperature stays above freezing
Show answer and explanation
Answer: A. Compressed air carries water and some compressor oil, which collect in the bottom of the tanks, B. With manual drain valves, you drain the tanks at the end of each day of driving, C. An alcohol evaporator puts alcohol into the system to reduce the risk of ice forming in the valves
Air pulled in by the compressor brings moisture with it, and that water plus a little oil settles in the bottom of the tanks. Water can freeze and shut a valve, which is how a brake system fails on a cold morning, so manual drains get opened at the end of every driving day. Alcohol helps with ice inside the valves but it does not remove the water sitting in the tank, so daily draining still applies unless the tanks drain automatically.
6. You push the pedal on an S-cam drum brake. What actually forces the brake shoes against the drum?
- A. Air pressure pushes directly on the back of each brake shoe
- B. A return spring releases and pulls the shoes outward
- C. Air pressure clamps a caliper around a rotor
- D. Air fills the brake chamber, the push rod moves the slack adjuster, and that twists the S-cam
Show answer and explanation
Answer: D. Air fills the brake chamber, the push rod moves the slack adjuster, and that twists the S-cam
Air only reaches the brake chamber. Everything after that is mechanical: the push rod extends, the slack adjuster turns the brake camshaft, and the S shaped cam spreads the shoes into the drum. That chain is why slack adjusters matter so much. If one moves more than about an inch by hand where the push rod attaches, that brake is not doing its share, and out of adjustment brakes are the most common defect found in roadside inspections.
7. Which statements about a dual air brake system are correct?
Select all that apply.
- A. It is two separate air brake systems, each with its own tanks and lines, worked by one set of controls
- B. Before driving you let the compressor build at least 100 psi in both the primary and secondary systems
- C. The warning light and buzzer must come on before pressure drops below 60 psi in either system
- D. If one system fails, the other stops the vehicle in the same distance as normal
- E. Both halves draw from a single shared air tank to save weight
Show answer and explanation
Answer: A. It is two separate air brake systems, each with its own tanks and lines, worked by one set of controls, B. Before driving you let the compressor build at least 100 psi in both the primary and secondary systems, C. The warning light and buzzer must come on before pressure drops below 60 psi in either system
One system typically serves the rear axles and the other the front, and they meet only at the controls, so a failure on one side does not take away all braking. Wait until both gauges read at least 100 psi before you move. Treat a warning in either half as a reason to stop, because a vehicle running on one half has lost part of its braking and will take noticeably longer to stop.
8. Complete the sentence.
If air keeps leaking away, the springs stop being held back and the spring brakes come fully on when tank pressure drops into the _____ range.
Options for blank 1: 5 to 15 psi, 20 to 45 psi, 50 to 60 psi, 80 to 100 psi
Show answer and explanation
Answer: 20 to 45 psi
Powerful springs are held off by air pressure while you drive. Take the air away and those springs apply the brakes mechanically, which is what lets one set of hardware serve as both the parking brake and the emergency brake. The manual gives 20 to 45 psi as the range, and your own vehicle applies at whatever figure the manufacturer set inside it. This is a last resort after a warning was ignored, never a plan for stopping.
9. On a newer air brake vehicle, the parking brake control is:
- A. A round red knob you turn clockwise
- B. A yellow, diamond shaped push pull knob
- C. A red, eight sided push pull knob
- D. A foot pedal to the left of the service brake
Show answer and explanation
Answer: B. A yellow, diamond shaped push pull knob
Pull the yellow diamond out to set the parking brakes and push it in to release them. On a tractor the red eight sided knob beside it is the trailer air supply, so the shape and color are how you tell them apart in a hurry. Never use the parking brake control to slow a moving vehicle, since spring brakes come on hard and do not act on every axle. Also leave it alone when the brakes are very hot from a grade, or wet in freezing weather, and chock the wheels instead.
10. Air brakes add a delay that hydraulic brakes do not have. At 55 mph on dry pavement, about how much distance does brake lag add?
- A. About 5 feet
- B. About 15 feet
- C. About 32 feet
- D. About 100 feet
Show answer and explanation
Answer: C. About 32 feet
It takes half a second or more for air to travel down the lines and apply the brakes, and at 55 mph the vehicle covers roughly 32 feet in that time before braking even begins. That is why total stopping distance for an air brake vehicle is four pieces added together: perception distance, reaction distance, brake lag distance and effective braking distance. For an average driver in good conditions the total at 55 mph runs over 300 feet, which is longer than a football field.
11. Which of these belong in the final air brake check of a pre-trip inspection?
Select all that apply.
- A. Build to full pressure, then shut the engine off, release the brakes and time the air loss
- B. Fan the pedal down and confirm the low air warning comes on before pressure drops below 60 psi
- C. Keep fanning until the parking brake knob pops out on its own
- D. Drain the tanks while the engine is running at full governor pressure
- E. Pump the pedal 20 times first to warm up the compressor
Show answer and explanation
Answer: A. Build to full pressure, then shut the engine off, release the brakes and time the air loss, B. Fan the pedal down and confirm the low air warning comes on before pressure drops below 60 psi, C. Keep fanning until the parking brake knob pops out on its own
You build to full pressure first, because every figure in the check is measured from a charged system. Then the order runs downward in pressure: static leakage with the engine off and the brakes released, then applied leakage, then the warning as you fan the pedal down, then the knob popping out lower still. The same check also covers governor cut in and cut out and the buildup rate, which should take pressure from 85 to 100 psi within 45 seconds at operating rpm on a dual system.
12. You are already in the right low gear on a long downgrade and using the snub braking method. When do you release the brakes?
- A. As soon as you feel the vehicle slow at all
- B. Only when you reach the bottom of the grade
- C. Once you smell the brakes getting hot
- D. When your speed is about five miles per hour below your safe speed
Show answer and explanation
Answer: D. When your speed is about five miles per hour below your safe speed
Apply firmly enough to feel a definite slowdown, hold it about three seconds until you are roughly five mph under your safe speed, then release and let the engine hold you until you drift back up to that speed, and repeat to the bottom. Light steady pressure the whole way down is what cooks the linings. Heat changes the lining chemically and expands the drums, so the shoes travel farther and press with less force, and once fade starts it feeds itself. That is why picking the gear before the hill matters more than anything you do with the pedal on it.
13. Complete the sentence.
Pressing and releasing the brake pedal when you do not need to lets air out of the system _____ than the compressor can replace it, so pumping the pedal to raise your air pressure actually _____ it.
Options for blank 1: faster, slower, more evenly
Options for blank 2: raises, lowers, has no effect on
Show answer and explanation
Answer: faster, lowers
Every release dumps some compressed air out of the system, and the compressor only refills the tanks at the rate the governor allows. Pumping is a habit carried over from hydraulic brakes, where it can help, and here it does the opposite. If pressure is falling on its own you have a leak, and the answer is to park, not to pump. The one time you fan the pedal on purpose is during the pre-trip check, when the whole point is to bleed pressure down.
14. System fully charged, engine off, service brakes released. How much air can a single vehicle lose in one minute and still pass?
- A. Less than 2 psi
- B. Less than 5 psi
- C. Less than 10 psi
- D. Any amount, as long as the compressor keeps up
Show answer and explanation
Answer: A. Less than 2 psi
Static leakage has to stay under 2 psi in a minute for a single vehicle and under 3 psi for a combination. Then you press and hold 90 psi or more, let the initial drop settle, and time it again: applied leakage allows under 3 psi a minute single and under 4 psi combination. The applied figures are higher because air has now moved into the chambers and lines, so there is more of the system able to leak.
15. A roadside inspector finds defective brakes. At what point does the brake condition alone put the vehicle out of service?
- A. As soon as any single brake is defective
- B. Only when more than half the brakes are defective
- C. When 20 percent or more of the brakes are defective
- D. Only when the vehicle cannot be stopped at all
Show answer and explanation
Answer: C. When 20 percent or more of the brakes are defective
The North American out of service criteria that inspectors work from take a vehicle or combination out of service when 20 percent or more of its brakes have a defect that impairs braking, and a fractional count rounds up, so a ten brake combination is out at two. Brakes out of adjustment are the most common defect found, which is exactly why the walkaround includes pulling on the slack adjusters. Other conditions can park a vehicle on their own, including an air loss rate over the limit or a low air warning that does not work.
What the CDL air brakes test is and why it matters
There is no separate air brakes license. The air brakes test is a knowledge test you take alongside the other CDL knowledge tests, and passing it is what keeps the air brake restriction off your license. Skip it, fail it, or take your skills test in a vehicle that does not have full air brakes, and the license comes back with an L restriction. That restriction bars you from driving any commercial motor vehicle equipped with air brakes, which rules out most of the tractor trailer and heavy truck work people get a CDL for in the first place. Clearing it later means going back to the DMV, passing the air brakes knowledge test, and retaking the skills test in an air brake vehicle if that is how you picked the restriction up.
The number of questions, the time limit and the passing score are set by your state, not by a national body. A short multiple choice test scored at 80 percent is common, but the only number you should trust is the one in your own state CDL handbook, so look it up before test day. The content is more predictable. States write the test from the air brakes section of their manual, which is close to the same document everywhere because it descends from a shared model manual.
That section expects you to know the three braking systems, the compressor and its governor, the tanks and why you drain them, the safety valve, the alcohol evaporator, the brake pedal, the foundation brakes with the S-cam and slack adjuster, dual air systems, the low air pressure warning, spring brakes, brake lag and stopping distance, how to brake on a long downgrade, and the air brake steps of the pre-trip inspection. Numbers matter here more than in most CDL sections. Some, like the 60 psi warning threshold, are set by regulation. Others, like the exact pressure at which a knob pops out, come from the manufacturer, and the manual gives a range. When your manual says the range is 20 to 45 psi, learn the range, not one number inside it.
How to use this sampler and how to study air brakes
These 15 questions stay inside the air brake system itself. They ask what the governor is doing, what happens mechanically when you push the pedal, why the warning light exists at the pressure it does, and what the inspection checks are really proving. They do not touch trailer coupling, the glad hands or the trailer air supply, which belong to the combination vehicles section and have their own sampler linked below.
Take it in one sitting without the manual open, then read every explanation, including on the questions you got right. Treat a miss as a topic rather than a fact. If you missed the leakage rate question and the governor question, sit down with the inspection sequence as a whole rather than memorizing two numbers, because the numbers only make sense in the order they are checked.
The part most people underprepare is the spoken pre-trip. Many states make you say the air brake check out loud during the skills test, in order, while the examiner watches. Reading about it is not the same as saying it, so practice it standing at a vehicle or at least out loud from memory: build to full pressure, shut down, chock and release, time the static leak, apply and time it again, then fan the pedal down and watch for the warning and then the knob popping out. Say the numbers as you go. The knowledge test and the skills test are asking about the same sequence from two directions.
How StudyPDF turns your CDL manual into full practice tests
Fifteen questions can show you which parts of the air brake system you cannot yet explain. They cannot get you ready on their own, and a generic bank does not know which state manual you are being tested from or which numbers your examiner will expect.
StudyPDF works from your material instead. Download your state CDL handbook, or use the air brakes packet your school handed out, upload it, and Bo, the study agent, builds full-length practice tests from those exact pages. Every question is grounded in your document, and every explanation points back to where the answer came from, so you can check the wording yourself when a number looks off between sources. You can regenerate fresh tests as often as you like, narrow one to a single area such as the pre-trip air brake check or spring brakes, and track which parts you keep missing.
You do not need a file to start. If your handbook is sitting in a truck somewhere, just name a topic instead, for example air brake inspection checks or braking on a long downgrade, and Bo writes a practice test from that. Starting is free.
Written by the StudyPDF team. Last updated 2026-08-19.