The studs that go through the ZF rear cover often seem to weep past the gasket face and along the threads and then past the nut . The open nut cannot fully seal it . As long your exposed thread length is compatible, you can cap the stud off with an Chromed acrorn nut and seal the base of the acorn nut with a copper crush washer or aviation gasket . Then the main leak path along the threads is fully encapsulated and sealed .
No more leaks. And the back of the ZF looks really nice !