I have a sump pump attached to my shower drain. The pump and float switch are working fine and are less than a year old. My problem is that once the water is pumped out there seems to be a back flow of water into the sump pump box. I don't think the hose to the thru-hull is clogged because I do see water being pumped out. About two weeks ago, my husband used a toilet plunger in the shower because the water was draining slow. It is an Attwood V750 pump and float. I checked the archive and I didn't see anything like this posted.Anyone have any idea about what to do to fix this? Thanks.