Car - AE86

Motor - Blacktop 20V

Use - Track

Problem - Brake pedal to the floor

Occurs/started when - After motorkhana run

Personal situation - At the last motorkhana after each run (about 20-30 sec flat out 1st/2nd gear) when trying to stop the brake pedal would go straight to the floor. Anyone shed some light on whey this could be occuring? Car has front and rear JDM disks with all rubbers lines replaced with braided ones. Master cylinder was rebuilt about 2 years ago and you can pump up the pedal with the car off and it feels soild. So I am thinking it has something to do with the vaccum. On the blacktop one of the original vaccum lines for the booster is used and just ran a rubber hose from the front of the motor while the other one is blocked off.

Thanks