I stuck to hydraulic pump back on the tractor today to see what the symptoms are. There appears to be some fluid flow, and the lift will track the position control lever very slowly as long as there isn't any weight on them. If I stand on the lift arms they go back down, even with the position control up. It would appear that the lift piston probably needs new seals.
I pulled the pump back off and opened it up and it looks ok. The pistons don't seem to have any notable scoring, the valve balls look good and the valve seats look ok as well.
I pulled the line set off so I can clean them out, and I guess I'll be pulling the lift cover so I can tear into that and also clean out the hydraulic sump fully. I did drain the fluid after the test and it didn't look bad, probably fairly recent. It will be replaced with UTF once I do seals on the piston and anything else I find wrong.
Also, I can't seem to budge the test plug, which is going to make it rather difficult to install a pressure gauge to see what pressure I'm getting.
Thoughts?