Our sump is attached to a pipe with a one way valve. The pipe goes aft and exits on the aft port side. The bilge pumps water through this pipe. The one way valve is to stop water left in the pipe flow backwards through the sump pump. Ours isn't quite so one-way. It's allowing water in the pipe to flow backwards to the sump pump and then in to the bilge where the pump dutifully pumps it back out again where it then leaks back and we have a sump pump thats continually cycling as a result it seems. About every 30 minutes, it kicks on again and then we wait for the backflow to kick it on again.
The dealer has an improved valve which they plan on fitting. More worrying is the same type of valve (at least to look at) is also on the toilet pipes heading up to the holding tank. If those leak, it's just nasty and sewage will back flow in to the toilet.