What is the function of the air compressor governor in an air brake system?

The AIR COMPRESSOR GOVERNOR is the regulating valve that controls the air compressor's loading and unloading cycles to maintain safe operating pressure in the air tanks. OPERATION: CUT-OUT PRESSURE — when system pressure reaches approximately 120-125 psi (the governor's upper limit), the governor signals the compressor to UNLOAD — it stops compressing air but continues to run (usually by opening an unloader valve that releases intake air rather than compressing it); CUT-IN PRESSURE — when system pressure drops to approximately 100 psi (the governor's lower limit), the governor signals the compressor to LOAD again — resuming compression. WHY THIS MATTERS FOR THE EXAM: If air pressure never builds above cut-in pressure, or builds above cut-out without stopping, the governor may be faulty; a compressor that won't unload causes excessive pressure buildup; safety (pressure relief) valve opens at approximately 150 psi to prevent catastrophic tank failure; air pressure should build from 85 to 100 psi in approximately 45 seconds on most vehicles — failure to build at this rate indicates a system problem. PRE-TRIP CHECK: With engine running, watch the pressure gauges — pressure should build to the cut-out range and then the compressor should cycle. Listen for the change in engine sound/load when the governor cuts out.