Periodic tests of joints and valves to ensure they aren't leaking or stuck, respectively. Servicing the air/water separator so that the stored air is dry rather than moisture laden, causing condensation and rusting inside the piping and storage tanks (see: stuck valves). Changing the compressor oil. Making sure no one stuck a metal blank on the emergency relief, making it useless. Hydrostatic load tests of the storage tanks. Things like that.