Re: naa hyrdaulic lever adjustment.
The cover oring just sits in cutout in the housing where the oil comes up, now as for the gasket it should be a thin paper gasket and best to get a ready made one so that you have the correct thickness.
Now for the bad news, yes it could be your pump and I would fro sure check that out first. BUT it could also be the metal line that carries the presurised oil back to the hydraulics leaking. It is a metal tube that goes though the bottom of the transmission and if there has been a lot of moisture in there then it will be rusted. I know because my 850 had this same issue and I had to split and replace the tube, actually both the supply and pressure had to be replaced.