I had purchased one of those gate valves last summer as when I would remove the cap, there was always about a quart of brownish fluid that would run out.
However, the first trip out after putting this on, I blew a tire and that gate valve became MIA. What I now wonder is if the tire blowing out ripped the gate valve off the sewer end, or if it fell off first and I ran over it with the tire that blew.
So, if you install one of these gate valves, make sure it is very tight. Because of the way it is constructed it is heavy on the one end and could have unscrewed itself.