Do I need to memorize the psi numbers?

Yes. The manual gives exact numbers for the governor, the low air warning, spring brakes, build-up time and leakage. Test questions and the pre-trip air brake check use them.

CDL Air Brakes Test: What It Covers and the Numbers

Q: Who has to take the CDL air brakes test?

The manual says you must take it if your vehicle has air brakes, including air over hydraulic brakes. If you will pull a trailer with air brakes, you also read the Combination Vehicles section. Check your state for its exact rules.

The CDL air brakes test is the knowledge test you take if you will drive a vehicle with air brakes. It covers how the system works, how to inspect it, and how to use it, and it leans hard on exact psi numbers.

Who needs the air brakes test?

The manual says you must take this test if your vehicle has air brakes. That includes air over hydraulic brakes. If you will pull a trailer with air brakes, you also read the Combination Vehicles section.

The manual also ties the license to the truck you test in. To be licensed to drive a vehicle with air brakes, you must take your skills test in a vehicle that has air brakes. Removing the air brake restriction takes a skills test, plus the knowledge tests.

The manual does not spell out the full restriction rules. Ask your state licensing office before you test in a truck without air brakes.

If the truck has air brakes, you also do the air brake check during the pre-trip. The manual warns that skipping any part of it means an automatic fail.

What the test covers

Section 5 of the manual has four parts:

Air brake parts. Compressor, governor, tanks, drains, safety valve, brake pedal, gauges, spring brakes, parking brake controls and ABS.
Dual air brake systems. Two separate systems, primary and secondary, with one set of controls.
Inspecting air brakes. The checks you add to the seven-step inspection, plus the final air brake check.
Using air brakes. Normal stops, ABS braking, emergency stops and stopping distance with brake lag.

Your state sets the number of questions and the passing score. Check your state's CDL manual.

The air brakes numbers to know cold

What	The number
Governor cut-out (compressor stops)	Around 125 psi
Governor cut-in (compressor starts)	Around 100 psi
Safety valve opens	Usually 150 psi
Low air pressure warning	Comes on before tank pressure falls below 60 psi
Large buses	Warning often at 80 to 85 psi
Spring brakes come fully on	20 to 45 psi (typically 20 to 30)
Pressure before driving a dual system	At least 100 psi in both systems
Build-up, dual system	85 to 100 psi within 45 seconds, engine at operating rpm
Build-up, single system (pre-1975)	50 to 90 psi within 3 minutes, engine at 600 to 900 rpm
Static leakage, single vehicle	Less than 2 psi in one minute
Static leakage, combination	Less than 3 psi in one minute
Leakage with brake applied, single vehicle	Not more than 3 psi in one minute
Leakage with brake applied, combination	Not more than 4 psi in one minute
Brake lag	One half second or more

On older vehicles the low air warning is one half the governor cut-out pressure. The warning is usually a red light, and a buzzer may come on too.

The two leakage tests

Drivers mix these two up.

Static test. Build the system to full charge, typically 125 psi. Turn off the engine. Release the parking brake by pushing the knob in. Time the pressure drop for one minute. The loss must be less than 2 psi for a single vehicle and less than 3 psi for a combination.

Applied test. Build pressure to governor cut-out and shut off the engine. Chock your wheels if needed and release the parking brake. On a combination, also release the tractor protection valve. Then fully apply the foot brake and hold it for one minute. A drop of more than 3 psi on a single vehicle, or more than 4 psi on a combination, is too much. Fix the leak before you drive.

The pre-trip version of both checks adds one detail. Start timing after the initial pressure drop of about 5 to 15 psi. For the applied test it also says to press the pedal firmly, at 90 psi or more.

The rest of the air brake check

After leakage, test the low air warning. Shut the engine off, turn on the electrical power, and step on and off the brake pedal to bring pressure down. The warning must come on before 60 psi.

Keep fanning the pedal. The parking brake valve should pop out between 20 and 45 psi, which puts the spring brakes on. Then restart the engine and check build-up time and the governor cut-in.

Do not push the brake pedal while the spring brakes are on. The manual says you could damage the brakes.

Try 10 exam-style questions built from the official CDL manual.

Traps on the test

Mixing up the leakage numbers. Static is 2 and 3. Applied is 3 and 4. Single is the smaller number each time.
Cut-in versus cut-out. Cut-in is the lower number, around 100. Cut-out is around 125.
Waiting for spring brakes. When the low air warning first comes on, the manual says to bring the vehicle to a safe stop right away, while you can still control the brakes. Do not wait for the spring brakes to come on.
Brake lag. Total stopping distance adds brake lag to perception, reaction and braking distance.

How to study it in a few days

Day one, learn the parts and what each one does. Day two, memorize the table above by writing it from memory. Day three, walk through the air brake check out loud in order. Then take practice tests and read why each wrong answer is wrong. If a number keeps slipping, put it on a card and carry it.