That valve by the sink in the bathroom . . .
Got stopped at a campground a few days ago, and was assaulted by a really terrible smell in the bathroom. Cut to the chase: I found it was that valve that allows air to flow when the sink is drained. ...
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If you use the drain plug and then try to drain the basin you will have a sink that has to 'burp' to drain properly since the purpose of the valve is to let air into the drain line and to prevent the drain from syphoning out which will give you back the smell through the sink drain. It's best to replace it.. they are pretty cheap.
