CDL Air Brakes Test: Sample Questions and Key Numbers
The CDL air brakes test is the knowledge exam that clears the “no air brakes” (L) restriction from a commercial license. It covers system parts, the PSI numbers examiners expect you to know cold, such as the low air pressure warning and spring brake activation range, and the air loss rate limits checked during the skills test. Most states require 80% correct to pass, per 49 CFR 383.135.
Why it matters: any driver who fails the air brake section of the knowledge test, or who takes the skills test in a vehicle without air brakes, gets the L restriction stamped on the CDL. That restriction blocks you from legally driving most straight trucks, tractors, and buses, since the majority of Class A and Class B equipment on the road runs air brakes. Clearing it means passing both the written air brake questions and, later, a skills test in an air brake vehicle.
What does the CDL air brakes test actually cover?
The test checks whether you understand the three air brake systems, service, parking, and emergency, and whether you know the specific pressure numbers that signal a problem. Expect questions on the air compressor and governor, dual air systems, low air pressure warning devices, spring brakes, and the air loss rate checks drivers perform before every trip. States write their own question sets, but nearly all of them follow the same AAMVA-model content found in state commercial driver manuals, so the underlying facts do not vary much from state to state, only the exact wording and, in a few cases, the exact PSI figures.
Why do you need to pass it? The “L” restriction
Under 49 CFR 383.95, a state must place an air brake restriction on your CLP or CDL if you fail the air brake portion of the knowledge test, or if you complete your skills test in a vehicle that is not air brake equipped. With the L restriction in place, you cannot legally operate any commercial motor vehicle fitted with air brakes, full or partial. Since most tractor-trailers, straight trucks over about 26,000 pounds, and full-size buses use air brakes, this restriction eliminates a large share of driving jobs. Removing it later requires passing the air brake knowledge test and a skills test performed in an air brake vehicle. You do not have to retake the general knowledge test to do it.
What PSI numbers do you need to know cold?
Examiners lean hard on pressure figures, because a driver who cannot recite them by feel is a driver who will not react in time on the road. The table below lists the numbers tested most often, drawn from state commercial driver manuals.
| Check | Typical range or figure | What it means |
|---|---|---|
| Air compressor cut-in / cut-out | About 100 psi cut-in, 125 psi cut-out | Governor stops the compressor at cut-out, restarts it at cut-in |
| Safety relief valve | Opens around 150 psi | Protects the first tank from over-pressure; if it releases air, get it fixed |
| Low air pressure warning | Must activate before the tank drops below 60 psi (California allows a 55 to 75 psi manufacturer range; large buses commonly signal at 80 to 85 psi) | Warns the driver before spring brakes start to apply |
| Spring brake (parking brake) activation | 20 to 45 psi, typically 20 to 30 psi | Springs mechanically apply the brakes once air pressure can no longer hold them back |
| Static air loss, engine off, brakes released | Under 2 psi/minute (single vehicle), under 3 psi/minute (combination of 2 vehicles) | Tests the system for leaks with no brake application |
| Applied air loss, engine off, brakes applied | No more than 3 psi/minute (single vehicle), no more than 4 psi/minute (combination of 2 vehicles) | Tests the system under a full brake application |
| Dual system air buildup | 85 to 100 psi within 45 seconds at operating RPM | Confirms the compressor can recharge both circuits fast enough |
Two things worth saying plainly here. First, the low air pressure warning number is the one place state manuals genuinely disagree, and it is worth checking your own state’s manual rather than assuming a single figure. The California DMV Commercial Driver Handbook states the warning “must come on when the air pressure in the tanks falls between 55 and 75 psi,” while the Pennsylvania Department of Transportation’s CDL manual states the warning “must come on before the air pressure in the tanks falls below 60 psi.” Both numbers satisfy the same underlying federal safety intent; the exact figure is set by each state’s licensing agency, following the AAMVA model informed by FMCSA’s regulations, not by one single national number. Second, the spring brake range (20 to 45 psi) and the air loss limits are consistent across the state manuals we checked, so those are safe to memorize as written above.
How is the air loss rate actually tested?
There are two separate leak checks, and mixing them up is a common wrong answer on the knowledge test. The static test happens with the engine off and the parking brake released: you time the pressure drop for one minute and it should lose less than 2 psi (single vehicle) or 3 psi (combination). The applied test happens right after, with the engine still off: you press the brake pedal and hold it for one minute, and the drop should not exceed 3 psi (single vehicle) or 4 psi (combination). Both figures come from the same CDL manuals cited above and match FMCSA’s underlying air brake performance standards. If either test shows more loss than that, the vehicle has a leak that needs repair before it goes back on the road.
What happens during the skills-test air brake check?
Before the road test, an examiner watches you perform an in-cab air brake check and narrate what you are doing. That typically means demonstrating and explaining the static leakage test, the applied leakage test, the low air pressure warning check (fanning the brake pedal until the warning activates), and the spring brake check (continuing to fan the pedal until the parking brake valve pops out on its own). You are expected to say the pressure figures out loud as you go, not just perform the mechanical steps silently. Examiners fail candidates who cannot state the numbers, even if the physical check was done correctly.
Sample questions and explained answers
These are original practice questions built from the manual content above, not copies of any state’s actual test bank.
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A driver fans the brake pedal to check the low air pressure warning. At what point must the warning activate, according to most state CDL manuals? Before the tank pressure drops below 60 psi (or the state-specific range, such as California’s 55 to 75 psi). The warning has to come on while there is still enough pressure left to stop safely.
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What causes spring brakes to apply automatically? Air pressure dropping into the 20 to 45 psi range. The springs are held back by air pressure during normal driving; once pressure falls too low, mechanical spring force pushes the brakes on whether the driver wants it or not.
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During a static air loss test on a single vehicle with the engine off and brakes released, what is the maximum acceptable pressure drop in one minute? Less than 2 psi. Anything higher points to a leak in the system.
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A combination vehicle loses 5 psi in one minute during the applied leakage test. Is that within limits? No. The limit for a combination vehicle under full brake application is no more than 4 psi per minute, so a 5 psi loss fails the check and the vehicle needs repair.
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What restriction appears on a CDL if a driver fails the air brake knowledge test? The L restriction, which bars the driver from operating any CMV equipped with air brakes, per 49 CFR 383.95.
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What is the usual air compressor governor cut-out pressure? Around 125 psi. The governor stops the compressor from adding more air once the tank reaches that level, and restarts it at roughly 100 psi (cut-in).
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A driver wants to remove the L restriction after already holding a valid CDL. What must they do? Pass the air brake knowledge test and complete the skills test in a vehicle equipped with a full air brake system. Retaking the general knowledge test is not required.
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What is the passing score for the CDL air brake knowledge test? 80% correct, the same minimum passing score FMCSA sets for every CDL knowledge test category under 49 CFR 383.135.
How is the test scored, and what if you fail?
FMCSA requires a minimum score of 80% correct answers on every CDL knowledge test category, air brakes included, under 49 CFR 383.135(a)(1). Most states run 20 to 25 air brake questions, so missing more than 4 or 5 typically means a fail, depending on the exact question count your state uses. A failed attempt does not block you from retesting, though most DMVs charge a retest fee and enforce a short waiting period. If you already hold a CDL with the L restriction and fail again, the restriction simply stays in place until you pass.
Run through the CDL Practice Hub CDL practice quiz to drill these PSI numbers before test day, and check the CDL Practice Hub CDL study guide for a full walkthrough of the air brake system if you want more than the pressure figures.
FAQ
Do all commercial trucks use air brakes? No. Many smaller trucks and some straight trucks under about 26,000 pounds use hydraulic brakes instead. Air brakes are standard on most tractors, larger straight trucks, and full-size buses, which is why the L restriction closes off a large share of driving jobs even though it does not apply to every CMV.
Can I take the air brakes test without ever having driven a truck with air brakes? Yes, the knowledge test is a written or computer-based exam on system facts and PSI figures. You do not need hands-on air brake experience to study for it, though hands-on practice helps with the later skills test, where you have to demonstrate the checks physically.
Is the air brakes test the same as an “endorsement”? Not quite. It is a knowledge test category tied to a restriction, not an added endorsement like hazmat or tanker. Passing it removes the L restriction rather than adding a new privilege to the license.
What happens if my low air warning doesn’t activate during the skills test check? The examiner treats that as an automatic failure of the vehicle inspection portion, since a non-functioning warning device means the vehicle cannot be driven legally until it is repaired. You would need to fix the issue and reschedule.
Do the PSI numbers change for buses? Some do. Large buses commonly have low air pressure warnings set to activate at a higher figure, around 80 to 85 psi, because bus air systems and passenger safety margins differ from freight trucks. Check your state’s manual for the exact bus figures if you are testing on a bus.