[jcslocum]

I don't have much to offer here and have watched this thread closely.

1) Does the pedal firm up and stop drifting if you increase the pressure a bit (not full stomp) when it begins to drift down??

2) You may have gotten a bad, new master. It happens, maybe change it again?

3) Get a used ABS pump from Woody to swap in and see if that fixes it.