I've encountered 3 families that their washing machine drain would flow back, due to grease clogs in the bigger sewer drain, below the washer drain, that had to be cleaned out every 6 months. So I installed washer backflow preventers for their washers, now their washer drains will never backflow, period, now the warm/hot water from the washer keeps the grease in the lower pipes moving along. They now haven't had clogged drains causing washer backflow for 2 yrs now. I've installed 4 backflow preventers now, and everyone of them have resolved the washer backflow issue.

Hi Wash and welcome to The Plumbing Page on AMHD.
You piggybacked on a 5 year old dead thread so I gave you one of your own. Before you post look in the upper left hand corner for the date..
We thank you for the update. By "backflow preventers" do you mean check valves? Back to you Tom