Update: Swapped the master cylinder, no dice. Pedal is still soft for the first half of its travel, then firmer for the second half. Still can push it to the floor with enough pressure. I haven't checked the pushrod clearance though, you need some $100 special tool for that, which I may get.

 

Dang. I was sure it would be the master cylinder.

 

Hopefully we will get some updates on this. My friend's car is getting the same problem at tracks between sessions, but mine is not having that.

 

Just for giggles and laughs, can you disconnect the booster and see what happens?

 

I assume you mean the vacuum line going to the booster?

 

I have the same issue, also running ATE fluid. I think the fluid can't handle it... haven't had time to flush it out with anything else yet.