Diff oil out the vent, why?

You may have a short tube vent bolt on a hump brake line. If the threads don't extend past the thickness of the housing enough the oil will flow down the hill up the vent bolt hole and out.

Tube vent bolt threads are shorter then the hump style to keep them off the axle. See if the threads extend past the housing on yours. They should. This stops the leak, Been there. **** don't flow up hill. Is your replacement brake line thicker the the original?