If it happens while you're at the track, you can bleed the brake lines and get past the boiled fluid. That'll help you, too.
You probably boiled the fluid during your first session, then, when you got things hot again during your second, it didn't take much to scorch it again and your pedal went away.
There are a couple of great "how to" articles around...
http://www.pelicanparts.com/techarticles/mult_bleed_brakes/mult_bleed_brakes.htm