CDL AIR BRAKE TEST

The CDL air brake test: 4 steps, the numbers, and what to say

The in-cab air brake check is the part of the pre-trip with exact numbers, and getting it wrong can end your test. Here are the four steps in order, the numbers to know, and the lines to say for each one.

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The numbers to know

Governor cut-out	120 to 140 PSI
Applied leak test, single vehicle (Class B)	No more than 3 PSI lost in 60 seconds
Applied leak test, combination (Class A)	No more than 4 PSI lost in 60 seconds
Low air warning (buzzer and light)	Before pressure drops below 60 PSI*
Spring brake pop-out	Between 45 and 20 PSI

* Some state manuals, like California's, say the warning must come on before 55 PSI. The federal standard for trucks built since 1975 is 60. Saying it came on before 60 is correct either way.

Step 1: Governor cut-out

Start the engine and let the air build. When the air dryer purges, the governor has cut out. Read both gauges.

Say this“My needle has stopped rising. The governor has cut out. My primary is 125 and my secondary is 125. This is a good test, because it cut out between 120 and 140 PSI.”

Watch the air brake test with the gauges moving

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Step 2: Applied leak test

Turn the engine off, turn the key back on, release the parking brake, and hold the brake pedal down for a full minute. Watch the gauges.

Say this“My primary is 118 and my secondary is 118. My sixty seconds begin now. I should not lose more than 4 PSI in sixty seconds.”

After the minute“My sixty seconds are up. I did not lose more than 4 PSI. This is a good test.”

Class B and other single vehicles: say 3 PSI instead of 4.

Step 3: Low air warning

Pump the brake pedal to bring the air down until the buzzer and warning light come on.

Say this“My primary is 62 and my secondary is 62. My buzzer and light came on before my air pressure dropped below 60 PSI. This is a good test.”

Step 4: Spring brake pop-out

Keep pumping until the parking brake valve (and the trailer valve on a combination) pops out on its own.

Say this“My primary is 38 and my secondary is 38. My spring brakes popped out between 45 and 20 PSI. This is a good test.”

Then: parking brake and service brake

Parking brake: with the parking brake set, put it in a low gear and gently pull against it. The truck should not move.
Service brake: release the parking brake, roll forward slowly, and press the brake. The truck should stop.
Automatic transmission: chock the wheels before the air brake test, and remove the chocks before the parking and service brake checks.

Questions

Is it 3 PSI or 4 PSI?

3 PSI for a single vehicle, like a Class B truck or bus. 4 PSI for a combination, like a Class A tractor-trailer. Mixing these up is one of the most common ways people fail.

Is the low air warning 55 or 60 PSI?

The federal standard for trucks built since 1975 is 60 PSI, and some state manuals list 55. Say the number on your gauge when the buzzer comes on, and that it came on before 60. That is correct in every state.

What happens if I skip the air brake check?

Many states treat a missing or incorrect air brake check as an automatic failure of the pre-trip. Do all four steps, in order, and say each one out loud.

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More pre-trip help:
What to say on the pre-trip California pre-trip inspection Free printable checklist Free checklist PDF

Study aid for the CDL skills test. It is not FMCSA-filed ELDT training and does not guarantee a passing score. The order follows the California checklist; check your state CDL manual for the order your examiner uses.