It is the "back pressure" in the discharge hose that forces the "lips" of a joker valve closed. They only fail when there is debris blocking them open; or, if they are worn. We have the same heads and only have to replace the joker valves about every 3-4 years as they become worn out (females aboard!). Something I have found of value is a material by the name of "Salt Away". We keep a squeeze bottle with a solution of this and fresh water in each head
. Once the bowl has been cleared of waste material, a good shot of the "Salt Away" solution is squeezed into the bowl, the flush lever fliped to empty, and the bowl pumped clear. Our heads and hoses have remained clear and trouble free ever since I discovered the stuff (2002).
It's available at West Marine
. While costly, one only needs a very dilute mixture. I use it in the heads, to keep the cooling
lines clear in our dinghy engine
and, after a long passage
, as a means of disolving away the salts on deck
without completely depleating our freshwater stock.