CDL Air-Brake Check Simulator
Walk the full air-brake check the way you perform it on the skills test. Each step shows how to do it, the exact pass criteria, and where the pressure threshold sits on the gauge.
Air-brake check
Step 1 of 8
Governor cut-in and cut-out
Do this
With the engine running, let air build to the governor cut-out, then fan it down and watch for cut-in.
It passes when
Compressor cuts out around 120–135 psi and cuts back in around 100–110 psi (check the manufacturer's range).
Compressor should cut out around 120–135 psi.
Why the test exists
Brake defects are the single biggest reason trucks get pulled off the road.
In the CVSA 2025 International Roadcheck (56,178 roadside inspections), brake-system defects were the leading vehicle out-of-service violation at 24.4% of all findings. Add defective brakes (16.7%) and brake problems made up over 40% of every vehicle put out of service, more than tires (21.4%) or any other system.
Source: CVSA 2025 International Roadcheck →How air brakes actually work
Air does the pushing, not fluid
A car uses brake fluid you press directly. A heavy truck uses compressed air as the force that pushes the brake shoes against the drums. An engine-driven compressor keeps the air tanks charged, and a governor stops it once pressure is high enough (cut-out) and restarts it when it drops (cut-in).
Springs hold the parking brake on
Spring brakes are held OFF by air pressure. If air pressure falls too low, powerful springs apply the brakes automatically. That is why a major air leak does not leave you with no brakes; it slams the spring brakes on. It is also why you never use the parking-brake valve while the spring brakes are already applied.
There is a built-in lag
Air takes time to travel through the lines, so there is a brake lag (around half a second) on top of your perception and reaction distance. At highway speed a loaded combination vehicle can need the length of a football field or more to stop.
Brake fade is the enemy on long grades
Brakes turn motion into heat. Ride them down a long hill and they overheat and fade. The fix is engine braking and a low gear chosen before the descent, using the brakes in firm, brief applications rather than holding them.
The air-brake check is part of the larger CDL pre-trip inspection trainer. Ready to test yourself? Take an air-brakes practice test for your state.