Air Supply, Compressor, and Reservoirs
The supply side is what makes everything else work: compressor, governor, dryer, and reservoirs. Maryland tests cut-in and cut-out pressures, build-up time, and the daily drain.
Compressor and governor
The compressor is engine-driven and has its own lubrication and cooling on many engines. The governor watches reservoir pressure: it cuts the compressor in at about 100 psi and out at about 120–125 psi. A safety valve on the reservoir vents at about 150 psi if the governor fails.
Reservoirs and moisture
The compressor's air enters the supply (wet) reservoir first, where most moisture and oil drop out; check valves then feed the dry reservoirs that supply the brake circuits. Drain all tanks daily - more often in humid or freezing weather. An air dryer with a desiccant cartridge removes vapor before it can freeze in lines; an alcohol evaporator is a secondary defense in cold climates.
Build-up check
With a fully charged system, run the engine at fast idle and watch the gauge climb from 85 to 100 psi; that should take 45 seconds or less on a dual-circuit system.
Worked example
The gauge rises to 100 psi, the compressor keeps running, and pressure climbs past 130 with no cut-out. Reasoning: the governor is not unloading the compressor. Stop and repair - the safety valve is the only remaining protection against over-pressurising the tanks.
Traps
Thinking the compressor can build pressure with the engine off; confusing cut-in (~100 psi) with cut-out (~120–125 psi); draining the wet tank only once a season instead of daily.