When I rebuilt the hydraulic cylinder on my 53 Jubilee project tractor, the rubber O ring and leather washer was completely gone. I could also see where the high pressure fluid had cut the edges of the piston.
I honed the cylinder and got a new style piston and a new rubber O ring and leather back-up washer. I also removed the unloading valve and spool and the control valve and cleaned them up good. Mine had the older style control valve without the O ring. I polished everything up good and put it all back together and it works pretty good. I have some minor hickuping but it does not affect the operation of the lift. The lift will also hold up my 5 foot brush hog for several hours.
Good luck on you hydraulic lift overhaul.
Rick H. Ga.