Different MC (re)manufacturers use different materials for the bales that hold down the cap, some of which are complete junk and don't exert enough force on the rubber seal to actually seal. Certainly any issues with the seal and/or the sealing surface won't help matters, so start there and make sure there is no corrosion, pitting, etc on the sealing surface. If it looks good, then inspect the gasket for any flaws or issues. You've already cycled through enough caps to more or less rule those out, leaving only the bale(s). I don't know of anywhere to just get the bale other than used, but I've seen plenty of folks with a hose clamp or two around the MC to keep it from leaking.