Sorry for the long post . . .
It all started about 3 months ago. I was using the pond scoop on the tractor ('48 8N) when I noticed that the lift became somewhat weak and would "creep" to the up position - even if I had the touch controller down. It would only do this when the scoop was empty because the added weight of the dirt was too much to allow it to creep up.
I removed the top plate to investigate and found a chunk of rubber (actually it was part of the piston seal) lodged in the inlet (or exhaust) valve of the pump. Obviously this prevented the hydraulics from working correctly. I took this opportunity to replace the piston seal and adjust the linkage using Zane's fixture.
Once everything was buttoned up, the position control worked flawlessly . . . until I put a load on the arms (like a finish mower, blade, etc . . . ). Now, it will lift about 1/2 way up, then stop lifting altogether unless I give it some gas to increase the pump speed.
Here is what I have looked at so far: 1. The piston/cylinder is not leaking. It will hold the mower up for a week. 2. I cannot feel any oil getting past the safety valve. 3. When running it and looking into the inspection cover, I don't see any leaks coming from anywhere . . . just a swirl of oil as you would expect.
My next move is to pull the pump and go through it to make sure something isn't awry. Aside from that, is there anything else that could cause this?