[jnaas2]

The style check valve Im talking about is made with the seat that the flapper seals against that is at a slight angle so it can be installed on a drain line with pitch and still be in the closed position without the help of a spring. The problem with a spring assisted valve is that you need extra water pressure to open it, and you may not get enough pressure from a gravity flow drain line to open it. If you go to a plumbing supply dealer they can get what you need