Cat 304C CR oil leak issue

I have a CAT 304C CR S/N OFPK02672 that blows oil out of the flat plate where the return filter goes. I have installed new CAT o ring with grease to retain it. When you operate the machine it blows oil out from the under the o ring and plate. The oil level is on the lowest mark on the sight glass. What can I do to correct this? Thanks in advance.