We are about to buy a house that has a horizontal run that 2 bathrooms on the second floor share. During the inspection of the house, when draining one of the bathtubs the other bathtub had a sewage back-up of black gunk and a horrible sewage smell. We are not sure if this is a major problem or a minor one. The current homeowners said they would fix the problem but we fear they may just unclog a blockage that is a reoccuring issue. Is this generally an easy fix or something I should stay away from? As a side note the house was built in the 50's and all walls are plaster.