Found relief valve after removing right rear wheel. It is located on the side of the loader control valve in the space between the bottom of the fender and sub frame, not on the bottom as shown in the diagrams. Removed, cleaned, and replaced the valve. No change. Also removed, cleaned and replaced check valve next to relief valve while I was there. No change. No unusual debris found in ports or fluid.
Additional observations:
1) The loader will go up and down with its full travel if you also curl the bucket one way or the other at the same time.
2) The 3 point lift moved to full up position during my manipulation of the loader, but I could not repeat the exact movements that allowed that to happen. It happened while I was paying attention the the loader movements. Strange.
3) Lowering the bucket in draft mode resets the lift of the loader, but it will stop half way up its full travel. Stopping the engine and restarting has the same effect on the loader as lowering in draft mode.
4) It does not seem that the loader valve linkages could be out of adjustment, but it is something to rule out.
It is due for a full hydraulic fluid and filter change. That will be my next step.
I will just keep ruling things out. Any ideas out there?