go back to a leather seal. If there was a bulge in the cylinder, then leather would take it up better than another sort of seal. You are usually advised to fit the washer dry and then put some oil in the tap and stand the gun up for 4 days or so. This makes it easier to fit than pre soaking. On some of the washers made (pretty much as one offs rather than making up a batch), and soaked, left them in a slightly smaller cylinder to form. I then left the screw that holds them slightly loose and then tightened it through the cocking lever slot.
You may have a leak at the cylinder end plug, but just go for checking the tap is aligned and sort out a leather piston washer for now.
The barrel is clean I take it?