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
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Also, on the cap that raised section of plastic on the cap is a vent, and it works best if that's at the highest point- ie, don't put that facing the front of the car, because when you stand on the brakes the fluid shift will put fluid out the vent.
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Like in this particular picture, your vents are facing mostly forward...
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