CDL Air Brakes Practice Test and Answers (Free Study Guide)

The CDL Air Brakes endorsement (sometimes called the "L" restriction-removal test) is required for any CMV equipped with an air brake system — which means almost every Class A combination vehicle and most Class B straight trucks. If you skip the Air Brakes test, your CDL will be issued with a restriction that prohibits you from driving any vehicle with air brakes. That restriction kills your job options.

This guide gives you free Air Brakes practice questions with answers, walks through the famous 7-step air brake pre-trip check in plain English, and covers the high-yield facts that show up on the real test.

What's on the CDL Air Brakes test?

The Air Brakes test typically has 25 multiple-choice questions and you need a score of around 80% to pass (which means missing no more than 5 questions). It's a separate written exam from General Knowledge, taken at the same DMV visit.

The questions cover:

• How an air brake system works — compressor, governor, reservoir, brake chambers, slack adjusters, S-cam
• The dual air brake system (primary and secondary) and why it exists
• Air pressure thresholds — 100-125 psi system pressure, 60 psi warning, parking-brake pop-out
• Service brakes vs. parking/emergency brakes vs. spring brakes
• The 7-step air brake pre-trip inspection
• Brake fade, brake adjustment, low-air warning signs
• What to do if your brakes fail or you experience low air pressure on the road

Free Air Brakes sample questions (with answers)

Try these eight before checking the answers. Be honest — guess only if you genuinely don't know.

1. The air compressor governor controls: A) When the air compressor will pump air into the reservoir tanks B) The maximum speed of the vehicle C) The temperature of the brake drums D) When the parking brake will engage

2. At what air pressure should the low air pressure warning light come on? A) 30 psi B) 60 psi C) 90 psi D) 100 psi

3. The parking brake on a tractor-trailer: A) Is operated by a yellow diamond-shaped knob in the cab B) Should be set whenever you park C) Is held in the released position by air pressure D) All of the above

4. Air-brake-equipped vehicles have a "spring brake" that: A) Engages automatically when air pressure drops below ~20-45 psi B) Engages only when you pull the brake pedal hard C) Should be used only in emergencies on the highway D) Is found on hydraulic brake systems

5. The maximum leakage rate for a single vehicle (truck or bus) with air brakes is: A) 1 psi per minute with the engine off B) 2 psi per minute C) 3 psi per minute D) 5 psi per minute

6. You should drain the air tanks (reservoirs): A) Once a week B) Daily C) Once a month D) Only when serviced

7. To test the parking brake, you should: A) Set the parking brake while still moving B) Stop, set the parking brake, and try to gently move the vehicle in low gear C) Pump the brake pedal three times D) Listen for an air leak

8. If your air brake pedal feels "spongy," it may indicate: A) Air in the hydraulic lines B) Water or oil contamination in the air system, or a brake out of adjustment C) The parking brake is set D) The compressor is over-pressurized

Answers: 1) A — the governor cycles the compressor on and off. 2) B — 60 psi. 3) D — all three are true. 4) A — spring brakes apply automatically when air drops, around 20-45 psi depending on the system. 5) B — 2 psi/min for a single vehicle (3 psi/min for combination). 6) B — daily, to remove water from compressed air. 7) B — set it and try to gently move forward. 8) B — water/oil contamination or out-of-adjustment brake. The other answer (A) is wrong because air-brake systems aren't hydraulic.

The 7-step air brake pre-trip check (memorize this)

This is the single most-tested section of the air brakes exam, and many states also require you to perform it during the road test. Memorize the order and what each step is checking.

1. Governor cut-in and cut-out pressures. Build pressure to ~125 psi; the compressor should cut out. Pump the brake to drop pressure; the governor should cut the compressor back in around 100 psi.
2. Air pressure leakage rate. With the engine off, fully apply the brake pedal and hold for 1 minute. Pressure shouldn't drop more than 2 psi for a single vehicle, 3 psi for a combination.
3. Low air pressure warning signal. Pump the pedal to drop pressure further. The warning light/buzzer should come on at or before 60 psi.
4. Spring brakes pop out. Continue pumping. The parking-brake control should pop out automatically between 20 and 45 psi, applying the spring brakes.
5. Air pressure rate of build-up. Restart the engine and run at a fast idle. Pressure should build from 85 to 100 psi in 45 seconds or less.
6. Test parking brake. Stop, set the parking brake, release the service brake, and try to gently drive forward in low gear. The vehicle should not move.
7. Test service brake. With air pressure normal, drive forward at about 5 mph and apply the brake firmly. The vehicle should stop straight without pulling, and the brake pedal should feel firm — not spongy.

A simple memory device: "Governor, Leakage, Warning, Pop-out, Build-up, Park, Service." Or use the acronym GL-WP-BPS ("Glad We Pass Brakes & Pass Safely").

High-yield air brake facts that show up over and over

• System pressure: 100-125 psi normal operating range
• Low air warning: comes on at or before 60 psi
• Spring brakes apply automatically: between 20-45 psi
• Maximum leakage: 2 psi/min single vehicle, 3 psi/min combination (engine off, brake applied)
• Drain reservoirs: daily, to remove water (water in the air system leads to frozen lines in winter and corrosion year-round)
• Stab braking (don't confuse with anti-lock): apply brakes fully, release when wheels lock, reapply — used only when ABS isn't available and you must stop quickly
• Brake fade is caused by overheated brake drums, most common on long downgrades. Use lower gears and engine braking to control speed instead of riding the brakes.

Common air brake test mistakes

1. Confusing "service brakes" with "spring brakes." Service brakes are what you push with the pedal. Spring brakes are the parking/emergency brakes that apply automatically when air pressure is lost.
2. Memorizing the 7-step order wrong. The order matters because each step builds on the previous one. Use the GL-WP-BPS acronym.
3. Forgetting to drain tanks daily. This is a real test question and a real maintenance practice. Skipping it leads to water-contaminated brake lines and is a frequent inspection violation.
4. Mixing up cut-in vs. cut-out pressure. Cut-out is the high pressure (~125 psi, compressor stops). Cut-in is the low pressure (~100 psi, compressor restarts).

Get the full Air Brakes prep package

Our 670-question CDL practice bank includes a complete dedicated Air Brakes section with all the question variants you'll encounter on the real test. Combined with our 210-page study guide eBook, it covers every air brake topic — including the seven-step pre-trip in detail with diagrams.

Unlock the full bundle for $49.95 →

CDL Air Brakes test FAQ

Do I need the Air Brakes endorsement if I'm only driving a Class B straight truck? Yes, if the truck has air brakes (most do). Without the endorsement, your CDL gets an "L" restriction prohibiting you from operating any air-brake-equipped CMV — which severely limits your job options.

Is the Air Brakes endorsement just a written test, or is there a road test too? The endorsement is added by passing the written Air Brakes exam. To remove the L restriction completely, you must also take your road skills test in a vehicle equipped with air brakes — the examiner will verify you can perform the 7-step inspection.

How many questions are on the Air Brakes test? Typically 25 questions; you need to get about 20 correct (80%).

Can I take Air Brakes the same day as General Knowledge? Yes — most states let you take all your written tests in a single visit. Plan accordingly.

Where can I download the air brakes section of the CDL manual as a PDF? The federal source is the FMCSA Model Commercial Driver's License Manual at fmcsa.dot.gov. Every state DMV also publishes its own version of the manual as a free PDF — search "[your state] CDL manual PDF" to download it.

Next steps

1. Take our free 10-question CDL practice quiz on the home page to baseline your CDL knowledge.
2. Memorize the 7-step pre-trip check above using the GL-WP-BPS acronym.
3. Drill the high-yield air brake facts until you can recite them without hesitation.
4. Get the full 670-question practice bank + 210-page eBook for $49.95 for comprehensive Air Brakes preparation.

The Air Brakes endorsement is one of the most useful CDL credentials you can hold — it expands your job options and pay range. With a few hours of focused study, it's very passable on the first attempt.