At what psi does the governor tell the compressor to cut in?

The governor of an air brake system is designed to control when the compressor begins to pump air into the system. Typically, the cut-in pressure—the pressure at which the governor activates the compressor—ranges between 85 psi and 100 psi in commercial and school pupil activity buses.

Choosing 85 psi as the cut-in pressure reflects a common standard for many air brake systems. This level ensures that there is sufficient air pressure to operate the brakes effectively while remaining within safety margins. When the system pressure drops to this level, the governor signals the compressor to start refilling the air tanks, thus maintaining adequate pressure for proper operation.

This setting is particularly important to ensure that the braking system is always ready to function when needed. Setting the governor cut-in at 85 psi balances the need for operational readiness without risking too frequent cycling of the compressor, which could lead to increased wear or overheating.