@mstas: I removed the filter and this screen for diagnostic purposes and there was no discernible difference in performance. Filter and screen ruled out.
@thefarmhand: All o-rings that are in or around the MCV have been replaced and all still look ok.
@JimB2: I don't know how this valve works, but it seems to me that if it were malfunctioning, I would lose steering pressure while lifting the loader but is it also possible I would lose pressure everywhere? When the pump got weak initially, I always still had steering and the loader would still go all the way up and down just off idle. It just didn't have any lifting power. Now, no hydraulics will work at all between Idle and at least half throttle. No steering, no 3pt hitch, no loader, no aux... the pto is good and strong though I have no idea how this is engaged.
@Harold Evans: The only way I know to bleed the air is to remove the flow divider valve and start the tractor until fluid flows continuously. Which I have done... is there more to it?