What does a slow leak in a PCP usually mean?

Slow leaks (a few hundred PSI per week or month) are most often caused by a tired o-ring at the fill port, the pressure gauge, or the burst disk seat - the high-cycle, easy-to-access seals.

A faster leak that empties the gun in a day usually points to the firing valve seat or main reservoir o-ring. Find the leak with soapy water or a leak-detection spray applied to suspect areas with the gun pressurized. Most leaks are user-fixable with a $5 o-ring kit; deeper internal leaks often require a qualified airgunsmith.