What is the purpose of the air compressor governor?

The governor controls when the air compressor will pump air into the storage tanks. When tank pressure rises to the 'cut-out' level (around 125 psi), the governor stops the compressor from pumping. When pressure falls to the 'cut-in' level (around 100 psi), the governor allows the compressor to start pumping again. This pressure cycling keeps the system within a safe operating range. The supply lines and brake chambers are controlled by the foot valve and the parking brake control, not the governor. The safety valve (spring-loaded, in the supply tank) releases air at around 150 psi if the governor fails. Spring brakes (the parking brake) apply automatically when air pressure drops below 20–45 psi, but that is a separate mechanism.