It's a straight 410, ignore the "D" if it's actually of that vintage.
Hose to what tank? I'm not aware of any hydraulic tank on that model, all the oil is held in the transmission case?
Does the transmission still work okay, drives forward and back properly? Transmission pump supplies oil to the inlet of the hydraulic pump.
Check the hydraulic filter, I think it's on the left side, there are two relief valves, one is a bypass for the filter element, not important but check it's free and the spring is good, the other is the pressure control for return oil to supercharge the hydraulic pump, if it's stuck open or the spring is broken, all the oil dumps back to the transmission case and there's no supercharge pressure for the hydraulic pump.