Well, I'm still fighting this. Bleed after every track day or 500 road miles. My dealer told me it was because of the hard track duty. Seriously, that's what the SM said!
Never mind that it does it after 500 road miles too.
Anyway, I searched herein and didn't find much except a bunch of people buying aftermarket slave pistons or slave cylinders. These are mainly folks that don't like the (high?) clutch lever effort. That's not my issue.
Has anybody considered excess temperatures at the slave causing brake fluid breakdown? If that evolved some vapor that didn't re-condense it would feel like air in the system and work its way up to the master and bleed just like air. I noticed the damn thing was way too hot to touch last week after a track session. I sure don't see any signs of fluid leaking anywhere. How can you get air in the system without having fluid come out somewhere?
I'm considering trying some Motul RBF 600 Dot 4 fluid. It is rated for nearly 600 deg F. Give it a good flush with that, then go abuse it at another track day !