^ what he said. i just got done having this problem, our slave cylinder setup is of (crappy) french design. my slave cylinder had a bypassing issue so it needed to be replaced, but... since you have to take the tranny off to get to the slave, you're gonna be better off replacing the clutch and resurfacing the flywheel while your in there. its a pain in the ***, and there isn't really much you can do about it other than fix it and wait for it to happen again.
next time i'd try removing the dust cap before filling the res though.lol