It is possible to get more than one reman or rebuilt master cylinder that leaks at the top. I keep an old tire tube around for times like this.
I might suggest you get something rubber, (do you still have the old MC) and cut a gasket to place on top of the existing one. It only needs to be at the rim where the top and sides meet.
I was able to stop a leaking MC by doing this a few years back that is still working today.
John