Hey Andy, No Fancy plumbing, no leaks, no silicone fluid? Bleed master again then re-bleed from longest line P/S rear working around D/S rear, P/S front and D/S front last. If still having a fade off the master cylinder must be bypassing between the cups or pulling air through a slave rubber(you would see a leak or moisture). Check all vacume points. I know you know all all of this? But if all is right I am not sure where to look either