Gehl Tandem drive pumps – leaking oil between them

So I've got a leak between these two tandem Eaton drive pumps, even when sitting it seeps, so I took them apart and checked shaft play on both and have none that I can determine. O ring was there, maybe was a little hard, but not distorted. Where they join there are matching holes for oil to pass from one to another, so does anyone else think that I may just have a simple O ring sealing problem? No funny stuff going on with machine driving that I can tell. Would hate to put it back together with a new O ring, only to find out it's an internal pressure issue or something.