Thanks MotorGeek, the mating surface is not pitted or scratched.
I figure it's just an old cap, after all it's the same one from when I bought the bike and I just figured it was time for a replacement.
I also figured if I've got to buy a cap it might as well be a higher pressure one, mine right now is 1.1 bar (16psi)
I have tested my system to 30 psi but am wary of running it that high long term unless someone has any experience.