My only experience with working on hydraulic components was when I replaced the seals in a wheel motor. I also ran the mower initially with the wheels off the ground. Another thing I read somewhere is to disable the engine from starting then crank it a few times to start driving the pumps just from the engine turning over.
On another note, I ran into a little snag on the restoration. I was starting to work on the steering controls. My intent was to use the components as a whole from the donor except the one obviously bent handle. After assembling the mechanism on the first one I found that it would not go to the fully dis-engaged position (handle completely outward). It was hanging up on the surrounding bracket (see pic). After dis-assembling everything and comparing side by side to my original part it was clear that the assembly that holds the handle was also bent. So I am having to dig deeper into the steering mechanism than I had planned and will end up using a mix of original and donor parts on the one side. While I was waiting (literally) for some paint to dry I also started working on the linkages that raise and lower the deck.