Can you overfill a PCP?

Yes, and you should not. Overfilling stresses the reservoir, regulator, gauge, and burst disk and can degrade or fail components.

On an unregulated gun, overfilling often produces lower velocity, not more, because the hammer cannot open the valve fully against excess pressure (this is called valve lock). A regulator does not let you safely fill the reservoir higher than the rated pressure; the regulator only controls downstream pressure. Stick to the marked max fill.