[Tommy]

My first guess would be overheated brake fluid causing bubbles in the system. The first solution is high boiling point brake fluid designed to handle the heat generated during track events. If you are already using that, I'd look next for unshielded brake lines running too close to headers and other very hot things. The next thing is to look for is ways to blow air on your brakes to help cool them quicker between applications. .. BTW, I always put in fresh brake fluid before every track day event. Most brake fluids attract water from the air over time and will boil it creating bubbles during hard brake use.