changed my brakes a few days ago, used a tool to twist/push the rear caliper piston back in, went to CSCS today, long story short, went out lapping, on the cooldown lap my brakes completely gave out. My foot was almost on the floor and I had little to no brakes. (keep in mind this was the cooldown lap, and not actually during racing)
I pulled off and checked the brakes, and lord behold the passanger's rear wheel was screwed from brake fluid hitting it. Thought I busted a line or something but I pulled the wheel for a closer look, and the leak was coming from the caliper piston itself.

When it cooled off, the pedal felt fine again, and I didn't need to drop it to the floor to get some braking force. (returned back to "normal" after some time.

now the question is, did the pedal drop to the floor because the fluid was boiling, or did that happen because of the brake piston leaking? Either way, I'm rebuilding the calipers, but I just want to know if I should stop using prestone dot4 and get some decent brake fluid?. (long story why I was using that and not something actually good)

 

A leak will cause a partial loss of brakes, I would say it wasn't your fluid boiling. Why it returned to normal is the real mystery ?. Leaks don't usually heal themselves. Good idea to rebuild the calipers.

 

and it wasn't leaking while I was actually under heavy braking, just when I started my cooldown lap, then when it cooled down it worked again? like what the hell is going on? bake booster + leak?
I just don't want to fix the leak thinking everything's all fine and dandy and go flying off the track at 170

 

I'll bet prestone Dot4 is fine but there are better options with higher wet and dry boiling points. JVF1 uses ATE superblue, I use Pentosin Dot4 (little bit lower temps so ATE is the better choice). You can also pony up for Motul 600c which is really nice stuff. I have used Motul 600c exclusively on my 2000 SiR for 10 years and it has been impressive.

Sorry I can't help on the braking except a rebuild may reveal the problem.

good luck

darcy