Brakes Normal.. then Grabbing really hard (almost locking), then super soft ??

I am rolling a BMW r1100s with Abs and servo assist.

When I first drove the bike it was fine with normal braking. after driving for say 20minutes, I touch front brakes and they are super grabby almost locking the front wheel! Later after more driving they were super soft requiring extreme pressure.

Before I disconnect the super sketchy bmw servo assist (and failure prone abs system)….

Anything else that could be causing this - old fluid? sticky calipers?

Nothing to lose by replacing the fluid, especially if it's a few seasons old, but the best place to find that answer is the Pelican Parts Forum. http://forums.pelicanparts.com/bmw-r1100s-r1200s-tech-forum/ I know there's a youtube video kicking around on bypassing the servo but I personally would prefer to have the ABS working.

I suspect the issue is not mechanical, but electronic.
When the bike warms up some electronic module also gets warm and starts to apply the brakes, just a little.

Find the fuse for the ABS/servo assist module, remove it, and see of the problem goes away.

BMW could learn a lot about reliable electronics from the Japanese.