I recently removed such a valve in when replacing the holding tank and septic system in my boat. It was probably about 15 years old and still worked, but I disassembled it to clean it (lots of calcium buildup). Ultimately, I decided to replace it with a new one because I could't get the smell out of the old one and I wasn't sure about the two O rings that probably should be replaced. When I got the new one, I did some experiments which might point to an installation issue with this valve. The valve uses a cone shaped piece that rotates in the body of the valve. It is held in place by the backing plate and a spring acting between the backing plate and the cone. The shaft for the handle goes through this spring and an O ring which seals it. It is possible to install the valve on the back side of a thin (1/4" or so) bulkhead and have the handle on the opposite side of the bulkhead. If the bulkhead were too thick, the handle would be pulled away from the valve, riding on the surface of the bulkead. This would cause the cone to pull against the spring, and open a gap between the cone and valve body. I was concerned about this because I wanted to install another on the back of a thicker bulkhead. I tested the valve bu filling one end with water and pulling out on the handle. Water leaked through the valve quite readily. I conclude that it is critical to install these things in a way that does not exert any force on the handle in the direction of the valve shaft on which the handle is mounted.
