At VIR Saturday I was racing in the 65 minute enduro. At about 20 minutes into the race I pitted for the mandatory 5-minute stop. Brakes were normal when slowing to the 40 mph pit limit and stopping at my spot. I rejoined the race after 5 minutes and when I got to turn 1 the pedal went to the floor. After backing into the wall I drove to the gap in the guardrail. Still no brake pedal. After sitting 20 minutes I checked the pedal again and it was normal (very firm). Pads are 75% good on all corners. No leaks seen on visual inspection. I have not opened the bleeders yet but I suspect I will find air bubbles.
I spoke at length with Bennett (a brake engineer) and he ruled-out boiled brake fluid, which was my first theory. He thinks that air entered the system somewhere, likely the "accordion" boot and/or piston seal. We are both perplexed by the self-healing of pedal pressure.
Any theories welcomed.