Bit late to this one but wondered if you’re still having the problem? I had similar on my track bike (not a blade btw) - changed fluid several times, took calipers apart and serviced, new seals in master cylinder and still i kept getting a long brake lever about 10 minutes into sessions. Had an idea to swap the discs from my wet wheels and instantly solved the issue. No idea what was wrong with the discs, they’re not warped or worn down past minimum thickness but that was the problem. Might be worth trying to get hold of some discs to try? Just an idea.