My boat has a small bilge pump in the sump for the shower. The initial problem was that the hose was so long to the thru-hull that once the pump shut off a good deal of water ran back into the sump. I installed a check valve about a foot from the pump. So then I created a second problem. 50% of the time air would be trapped between the pump and the check valve, and the valve would not open, and of course the sump wouldn't drain. I solved the air problem by drilling a 1/8" hole in the side of the hose just ahead of the valve. When the pump turns on, the air has a place to escape and then a bit of spray will come out of the hole into the sump. I noticed that a side benefit of the spray is that it stirs up any residue in the bottom of the sump which then gets pumped out. Seems to be working for me. Maybe a solution for others.
JerryA
JerryA