De gas the gun, remove the foster fitting and examine the plunger seal. I have never had mine out of my Marauder, but they normally have an O ring under a plunger that seals them. It could just be a small piece of crud that is keeping it from sealing or it might need a new O ring that you can find at a hardware or auto parts store. PA has them for $15.99 in stock. I would order from Crosman.