Hi...Just bought an old farm house built in 1949 and have found out there's a problem with the plumbing. At first I thought the shower/tub was just draining slow so we plunged it, only to discover it was then backing up into the other bathroom shower. The toilets seem to be okay but one of the bathroom sinks are also included in this backup problem. This seems like a very expensive problem to me. Are there any recomendations on how to best approach this??