[jhv48]

So, if I get this right.

Clutch operates fine when cold but fails when good and hot?

No visible leaks probably means one of two things.

1. Master cylinder or slave has an internal leak when the seals get good and hot allowing the hot (perhaps boiling) fluid to slip past the seals.
2. The fluid is boiling in the lines causing a loss in pressure. Try running with the electric fans on continuously. Does the problem persist?

Drain the hydraulic fluid first and see what color it is. If dark brown/black, it probably has been burned. Change to a high temp DOT 4 fluid such as Wilwood 570. If it has boiled, time for some custom heat shields.

Now drive it and see if the problem recurs. If so, then climb under the car (while the thing is still good and hot) and have a friend depress the clutch pedal. Observe the travel that the clutch fork makes (or doesn't). Is it the same as when dead cold? If not, then the M/C or slave are losing pressure internally. That will necessitate replacing first one then the other. Start with the one closest to the heat sources (headers). If that makes no change, then go to the other one. One or the other will prove faulty.

And this is not ONLY a BDR problem. I've been chasing my heat devils in an SPF for the last few weeks.