Help! I moved into my mothers home after she passed way. The home is about 35 years old. As the washing machine is pumping out, it backs up into the drain tube and overflows. My brother advised this problem has existed a long time. I have tried everything, plumbers snake to ensure no blockage, vent clear, installed a diaphram to allow more venting, all to no avail. I am out of ideas. The drain size is 1 1/2 inches. Any ideas??