as 66coupe289 stated could be lots of things. You mentioned having the rotors off doing wheel bearings. maybe you smudged grease on the pad without knowing ? maybe the pads don't move freely as they should, maybe the master cylinder has an issue, maybe the pads are "glazed over" , The Granada was a bigger/heavier car than the Mustang and they stopped that ok ...OK well, what might be wrong with them? Master cylinder or pads come to mind.
I adjusted the proportioning valve fully clockwise which increases front wheel bias and although better it still fells like only the rears are doing any braking. Like trying to stop using the parking brake. Any thoughts on that?