Should I store a PCP empty or pressurized?

Store a PCP pressurized, ideally at or near its working fill pressure or at the regulator setpoint.

The pressure keeps internal o-rings seated and seals from drying out. Long-term empty storage is the most reliable way to develop slow leaks the next time you fill the gun. If you must travel and absolutely cannot ship a pressurized airgun, fully degas it per the manufacturer's procedure rather than just bleeding partly.