Rebuild kits with O-rings and seals can sometimes be found for name brand jacks. As for replacing the rubber plugs, I will note that the 50 year old Blackhawk jack had a metal plug in a threaded hole. I like that.
The one critical thing that is not shown in the diagram below is the release valve. It generally provides a "bypass" path from the lift cylinder back to the fluid reservoir.
Tom's Auto repair pages / email@example.com