Kevin, check the groove and the crush rib on the PM55 tank halves carefully, you may find something that is preventing the tank gasket from sealing properly. I have used cork gaskets, with and without sealer, square o-ring material, and the Viton gaskets from Sawzilla and I definitely prefer the Viton. They will swell when they are soaked with fuel but as M S pointed out, that creates a tighter seal. You cannot reassemble one if it is swollen but give it a day or two and they will return to normal size.
Mark
Mark