Coyled:
Lot of good dicussion. Don't think you've have got to the root cause of going spongy on the track.
It is caused by one of two things; your changing a drop of water to gas at about 212 deg or your exceeding the boiling point of the fluid somewhere(400-500 deg).
First: BTSnake is right you can have NO!!! water is the system. This is the most common cause. Everyone has experinced a soft pedal and upon bleeding found "one" 1/8 in bubble, bleeding that out fixes the pedal. One drop of water, when flashed to steam, becomes and stays a bubble abut 1/2 in long totally destroying the brakes.
1. The MC should be shrouded from the header with a heat shield, this stops the radiate energy from getting too and heating up the MC; low problablity. MC is big and has a lots of mass and it's 8- 12 in from headers.
2. Overheating the front calibre, exceeding the 500 deg and boiling the fluid in the piston. This is common when you really use brakes hard with race tires. Solution- ducts; everyone was concentrating on rotor cooling, who cares! rotors go to 1500 deg and glow but cool rapidly when brakes are released. The key is that you cant have the caliber get over the boiling point of the fluid inside; thats the priority of the cooling duct; most race cars duct to both. But, the spongy system is caused by the caliber overheating and flashing a bubble inside the caliber.
3. The most likely, your rear line probably turns up from your floor pan and passed within 1-2 in of a header pipe. On the track the radiate energy from the pipe is 10-20 times that of street driving. The most likey is that you are exceeding 400 degs locally in that region and flashing a bubble in the brake line.
My solution: run the line in a mesh/alum/silicon tube designed for that purpose, attach the line right agaisnt the glass foot box, then put a standoff heat shield between the line and header. My heat shield is stood off by a thick nut that attaches the heat shield. Leave the shield open at the top and bottom so the hot air can raise and cool the area behind the shield
gn
