I have 60 year old house, that is connected to city sewage. Every time we run our dishwasher, or flush our toilet our basement utility sink fills with dirty water. How do i stop this from happening? The (I believe) main outlet pipe has all up stairs draiage plugged in above the basement utility sink, I don't know if this simply needs a back flow stop or if this could be a clogged sewer line. How can I tell, and how can I fix this. Please Help!