CDL Air Brakes Test: What You Need to Know

What the air brakes test covers

The test covers seven areas that 49 CFR 383.111(b) lists for anyone operating a vehicle with air brakes. That is the regulation behind your state's questions, so it is a reliable map of what to study. The skills side is separate. When you take the skills test in an air brake vehicle, 383.113(a)(2) adds hands-on air brake inspection skills, covered below.

• General air brake system nomenclature (the parts and what each one does).
• The dangers of a contaminated air supply: dirt, moisture and oil.
• What happens when air lines between the power unit and trailer are severed or disconnected.
• What low air pressure readings mean.
• Pre-trip inspection procedures, including automatic fail-safe devices, system monitoring devices and low pressure warning alarms.
• En route and post-trip inspections, including air loss tests with and without the engine running and the pressures at which the low air warning and tractor protection valve should activate.
• General operating practices: proper braking technique, antilock brakes, emergency stops and the parking brake.

When you have to take it, and what score passes

You take the air brakes test if the vehicle you will drive has air brakes. The Virginia DMV's CDL manual says this plainly and includes air over hydraulic brakes. That matches 49 CFR 383.95(a)(2), which defines air brakes as any braking system operating fully or partially on the air brake principle. The passing score is the same as for every other knowledge test: at least 80 percent of the questions correct (383.135(a)(1)). It is scored separately, so a strong general knowledge result does not cover a weak air brakes result. Question counts and fees are set by your state.

What happens if you fail or skip it

If you score under 80 percent on the air brake portion and a CLP or CDL is issued anyway, an air brake restriction must be placed on it (49 CFR 383.135(a)(2) and 383.95(a)(1)). The Virginia DMV manual lists this as restriction L, and says it applies if you do not take the air brake test or fail it. The restriction bars you from operating a commercial vehicle with any type of air brakes. The same restriction goes on your license if you take the skills test in a vehicle that has no air brakes (383.135(b)(3)). Either way it rules out most tractor-trailers and many straight trucks. It can be removed later. 383.135(b)(7) says you must pass the air brake knowledge test and successfully perform the air brake pre-trip inspection, and the state may also require a modified skills test. Your state sets the process and fees.

The pressure figures to know

These come from the state CDL manuals, which adapt the national model manual. They are described as approximate, and the manuals say to check the manufacturer's specifications for the vehicle in front of you. Figures below are from the Virginia DMV CDL Manual (Modernized Testing System, March 2025) and the North Carolina DMV Commercial Driver License Manual, which agree except where noted.

• Governor cut-out: the compressor stops pumping at about 125 psi.
• Governor cut-in: the compressor starts pumping again at about 100 psi.
• Low air pressure warning: must come on before pressure in the tank falls below 55 psi in the current Virginia manual. The North Carolina manual, an older edition, says 60 psi. Use your own state's manual for the number it tests.
• Spring brakes: the parking brake valve pops out and the spring brakes apply when pressure falls to between 20 and 45 psi.
• Pressure build-up: 85 to 100 psi within 45 seconds in a dual air system, with the engine at operating rpm.
• Leakage with the engine off and parking brake released: less than 2 psi in one minute for a single vehicle, less than 3 psi for a combination.

The air brake system check, step by step

The manual puts these checks in Step 7 of the seven-step inspection, in place of the hydraulic brake check. The skills test (383.113(a)(2)) expects you to be able to do this and explain it. Virginia's skills-test section says failing to explain and demonstrate all four parts of the air brake check correctly is an automatic fail of the vehicle inspection test.

• Check the governor. Run the engine at fast idle and watch the gauge stop rising at cut-out. Then step on and off the brake to bring pressure down and confirm the compressor cuts in.
• Test air leakage. With a fully charged system (typically 125 psi), shut off the engine, release the parking brake and time the pressure drop. Then fully apply the foot brake for one minute. The manual's limit for that test is no more than 3 psi for a single vehicle or 4 psi for a combination.
• Test the low pressure warning. With the engine off and electrical power on, step on and off the brake to reduce pressure and note where the warning comes on.
• Check that spring brakes come on automatically. Keep fanning the pressure down. The parking brake valve (and the tractor protection valve on a combination) should pop out in the manufacturer's 20 to 45 psi range.
• Check the pressure build-up rate, then test the parking brake and the service brakes by moving forward slowly at about 5 mph and braking firmly.