A picture of what you describe would be helpfull. It’s been over 6 years since I sold my ST4S and cannot for the life of me remember what the fuel cal mechanism looks like. Andy
Make sure you get the correct fuel resistant material like Viton. I replaced the 'rubber' washer on my Guzzi fuel tank as it was worn/squashed so thin that fuel would ooze out under braking. I bought a sheet of suitable thickness, claimed thickness "fuel resistant" stuff, fitted it and it seemed fine but the next time I filled up it was the consistency of snot. Bought a sheet of viton, made a new one and it's been fine since.
Here’s a Heath Robinson fix, take the cap off and give the rubber a real good clean, get some thinners on the rubber for a while if you’re lucky it will soften it, cut a circle of 1mm rubber sheet an cut it (like a spring washer is cut if that makes sense) so that you can fit it between the original seal and the steel petrol cap, that will have the effect of thickening the original seal, might work might not!
We have a product here called belt grip made by CRC which is very good at softening rubber . Maybe worth cleaning everything up and if you can get some give the seal a good spray with it.
