don't push the air nozzle against the cylinder head when you test for compression, and the piston should move forward very slowly. It should leak a little bit but not much at all, if it does then you need a new or different brand of air nozzle.
Try reinstalling your hopup rubber, make sure you use silicone lube on the OUTSIDE of the hopup rubber to make sure it doesnt catch or stick to anything while it's sliding into the hopup chamber
