Clik, this was August in California so not a case of low temps. It was purely a case of lower atmospheric pressure releasing the air trapped in the brake fluid. It was a real learning experience. Furthermore, during the return trip over the mountains the brake pedal stayed all good. Why, because the entrained air had already escaped. It was an interesting situation for sure. Oh and as far as mixing fluids, the chassis was completely rebuild and everything was dry before the silicone was added. I had another situation with the DOT 5 fluid. I got invited to "play" at Laguna Seca and my brake fluid got hot enough that I lost the brake pedal entirely. As I lapped the course I could feel the pedal get lower and lower until I had to pit due to lost brakes. After a couple hours cool down the pedal came back. Needless to say fellas I run Castrol LMA, which is DOT 4, now. Oh yeah, to the OP I agree with a couple of other posters, you have a leak. The fluid does boil but geez, you'd have lost your pedal before it would vaporize out of the system.
Even though it can boil, it sounds like you have a leak somewhere. A small leak can be hard to find sometimes. Take a paper towel and make sure all your lines are clean and dry....take a drive and check them again, master cylinder, etc....if you get a moist towel...you found your leak......even if it is boiling, I seriously doubt that that is your fluid loss problem. I busted a break line a month or so back....just a weak point in the line over many years finally gave way....glad it happened in my driveway! Be careful, good brakes are important!!!!
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