what

[WOLLFFY]

What would cause a leak out the back end of a classic valve?

[flyingpootang]

Piston o-ring cut.

[Coralis]

velocity turned up too high will also cause this

[TrracerAce]

Mine does this once in a blue moon when I gas it up, cycling it once or twice always fixes it.

Though it often overpressurizes upon gas-up and shoots off that first shot at ~450 fps.