I doubt there's any "boil over" involved unless you've experienced a ton of brake fade when driving. The gaskets that come on those master cylinders are kinda lousy. If you open and close them a bunch (like during the bleeding process) it seems like they start to leak.
You can buy replacement caps and gaskets for them, I did that with mine and haven't had any issues since. Dorman #42044 for caps, #42081 for gaskets
