The other problem is that the whole master cylinder assembly is bolted directly to the engine cases. It's not just the heat from the exhausts you've got to worry about.
I've tried "insulating" the assembly from the cases with little/no success.

I'd not thought of the bleed nipple letting air in. A very valid point and worth a try :cool:

Quite a fascinating topic!

It would be quite a coup if some DSC member could design an improved rear brake.
So many ideas have been mooted: the fluid overheating, not enough fluid in the system, relocating the master cylinder, Aprillia calipers, leaking bleed nipples etc etc.

I guess many of them are partial causes. I certainly think Go Falfer could be on to something about the nipples. They always seem to weep a few bubbles in the tube when I connect the mityvac to them.
But then, I can't be sure it's not leaking into the tube past the outside of the nipple from the atmosphere.
At least, that's what I assumed before.

Brembo had some issues with the bleed nipples when we were working on some bikes that were out of spec. The location/placement of the master does not help the sitaution either with all the heat transfer.