CDL Air Brakes Test: What's on It and How to Pass

Ask a room of CDL students which knowledge test worried them most, and air brakes wins almost every time. The general knowledge test is broad but intuitive. The CDL air brakes test is narrow and technical: it asks for specific pressures, leak rates and component names that you've probably never seen outside a truck. The good news is that the material is finite. Learn how the system works and memorize about a dozen numbers, and this becomes one of the more predictable tests you'll take.

Why the CDL air brakes test matters (the L restriction)

Most Class A tractors and many Class B trucks and buses use air brakes. If you don't pass the air brakes knowledge test, or you take your skills test in a vehicle without full air brakes, your license gets an air brake restriction (usually shown as "L"). That restriction means you can't legally drive a commercial vehicle equipped with air brakes.

For most trucking jobs, an L restriction is a deal-breaker. So even though it's technically a separate test, treat air brakes as mandatory unless you're certain your career will never involve an air-braked vehicle.

Test format: the number of questions is set by your state; most use about 25. Federal rules require a score of at least 80% on each CDL knowledge test (49 CFR Part 383). Check your state's CDL manual for its exact count and retake rules. The federal requirements are summarized on the FMCSA's CDL page.

How air brakes work: the three systems

Air brakes are really three braking systems sharing one air supply:

1. Service brakes apply and release when you press and release the brake pedal.
2. Parking brakes hold the vehicle when parked, using spring force.
3. Emergency brakes use parts of the service and parking systems to stop the vehicle if the main system fails.

The main components you'll be asked about:

• Air compressor pumps air into the storage tanks. It's driven by the engine.
• Governor controls when the compressor pumps air into the tanks (cut-in) and when it stops (cut-out).
• Air storage tanks hold the compressed air, with enough reserve for several brake applications if the compressor stops.
• Drain valves release water and oil that collect in the tanks. Manual drains should be opened at the end of each day.
• Safety valve protects the tanks from over-pressure, usually opening around 150 psi.
• Brake chambers, pushrods, slack adjusters and S-cams convert air pressure into the mechanical force that pushes the brake shoes against the drums.
• Supply pressure gauge shows tank pressure; the application pressure gauge (on some vehicles) shows how much pressure you're applying to the brakes.

Numbers to memorize

These are the figures that appear most often. They're taken from the FMCSA model CDL manual most states base their manuals on, but some states use slightly different values, so if your state manual disagrees, your state manual wins.

Two of those trip people up more than the others. The leak rates are easy to mix up because applied and released numbers are close. Remember: applied is always one more than released. And the spring brake range gets confused with the low air warning. The warning comes on first (by 60 psi); spring brakes come on later, as pressure falls into the 20 to 45 psi range.

Dual air systems and spring brakes

Most modern heavy vehicles have dual air brake systems: a primary and a secondary system, each with its own tanks, hoses and lines. Usually the primary system operates the brakes on the rear axle or axles, and the secondary operates the front axle (and sometimes one rear axle). If one system fails, the other still provides some braking.

Spring brakes are the safety net. During normal driving, air pressure holds powerful springs back. When you set the parking brake, or if air pressure leaks away, the springs apply the brakes. That's why a serious air leak eventually locks up the brakes.

Common test questions here: what happens if one system fails (stop safely as soon as possible), and why you should not use the parking brake if the brakes are very hot (they can be damaged) or frozen.

The 7-step air brake check

The in-cab air brake check is part of your skills test, and its steps are tested in knowledge questions too. The standard sequence:

1. Test the low pressure warning signal. With the engine off and enough pressure, fan the brake pedal to lower pressure. The warning should come on before 60 psi.
2. Check that spring brakes come on automatically. Keep fanning. The parking brake valve should pop out between 20 and 45 psi.
3. Check the rate of air pressure build-up. Start the engine. Pressure should go from 85 to 100 psi within 45 seconds in a dual system.
4. Test the air leakage rate. Shut off the engine, release the parking brake, apply and hold the service brake, and watch the gauge for one minute.
5. Check the governor cut-in and cut-out. Watch for cut-out around 125 psi, then fan to see where the compressor cuts in, around 100 psi.
6. Test the parking brake. Set it and gently pull against it in a low gear to make sure it holds.
7. Test the service brakes. Move forward slowly and apply the brakes firmly. The vehicle should stop without pulling to either side.

For how this fits into the full vehicle inspection, see our guide to the CDL pre-trip inspection.

Common air brake test traps

The questions that catch people usually involve:

• "Brake lag." Air brakes take a fraction of a second to work, which adds distance to every stop. Expect a question about why your stopping distance is longer than in a car.
• Brake fade. Overusing the brakes on a downgrade heats the drums, which reduces braking power. The right technique is a low gear plus steady braking to about 5 mph below your safe speed, then releasing.
• What to do when the low air warning comes on. Stop and park safely as soon as possible; don't keep driving.
• Draining tanks. Water and oil in the tanks can freeze or damage the brakes. Drain manually operated tanks at the end of each driving day.
• Front brake myths. Some drivers believe front brakes cause skids on slick roads. The manual says the opposite: front-wheel brakes are good under all conditions, and tests show front-wheel skids from braking are not likely even on ice.

Our CDL practice sets include air brake questions alongside the CDL general knowledge test material. If you're still at the permit stage, start with how to get a CDL permit, then run the free CDL practice tests until you're scoring well above 80%.

Frequently asked questions

How many questions are on the CDL air brakes test?

It depends on your state, but most use about 25 questions. You need at least 80% to pass under federal rules.

What happens if you fail the air brakes test?

You can retake it according to your state's rules. If you don't pass and still get your CDL, you'll have an air brake restriction that bars you from driving commercial vehicles equipped with air brakes.

What PSI should the governor cut out?

The model CDL manual gives about 125 psi for cut-out and about 100 psi for cut-in. Check your state manual in case it lists slightly different numbers.

Do I need air brakes for Class B?

Only if you want to drive Class B vehicles with air brakes, which includes many dump trucks and buses. Without the test, you'll carry the air brake restriction.