For any leaks within the gearbox, you have to strip the gearbox down and test the compression of the piston in the cylinder with the cylinder head attached and the nozzle in place. Stick your finger over the nozzle and try pushing the piston in. Once it passes the cylinder window (if there is one), it should be almost solid. If it is not, it is one of:
Piston Head O ring
Cylinder head O ring
Nozzle (if it does not have an O ring, get one with an O ring).
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You can also check the seal between the nozzle and the hop rubber; make sure that there are no BBs in the chamber, turn the gun upside down, place a small piece of paper over the feed tube and pull the trigger. If the paper flies into the air, the nozzle is not fully entering the hop rubber lips; they could be worn or the nozzle could be too short.