From what I can remember the reservoir cap has two fingers that lock onto some threads on the reservoir itself. I ended up stripping the threads on my original reservoir and the cap would never seat down tightly after that. I never really lost fluid, but then again I don't really do any "spirited" driving. Maybe check the threads that the cap screws onto and see if they are intact. I have NO idea how to fix it, though, so I don't guess I'm much help.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless