My first thought was that you had a major leak in the lift piston. Since you have checked that and it appears OK, I would check the pump further
Is the lift jerky or smooth? If its jerky, I would try bleeding the pump by the pipe plug on the top - front of the pump. If you are up for a task, pump in a clear plastic hose and run it to the fill hole under the seat. Start the tractor. If you see bubbles running though the plastic tube, you have a suction side air leak. If you just see oil, I'm thinking you might have a few broken or worn check valve springs inside the pump.
If it were mine, I'd bleed that pump and see if it improves things. Just break the pump bleed plug loose (just a bit) and run the tractor at slow idle. Loosen the pipe plug just enough for oil to flow. You may see bubbles. Once its only oil, tighten the plug and check the lift. Be careful, if the plug comes out with the tractor running, you'll be moping up oil all over the barn.
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