The joker valves start leaking back in very short time in my experience and probably not unexpectedly so given their simple construction. They are particularly prone to losing their sealing ability if someone pumps hard against a blockage so building up back pressure which then inverts the joker valve inside out when pumping is stopped - it is the pushed right way around again next time normally pumped with solids (hopefully
In our own case we don't worry about the condition of the joker valve too much as we took the inlet to the holding tank into the top of the tank. We take care to pump enough so that there is only clean sea water left in the pipe lift
to the top of the holding tank or, if discharging overboard
, to the top of the siphon break.
For us, if a new joker valve is not fitted, we get about 1/3 of the bowl fill with sea water back flow in the worst case (worst case being from the lift
in the antisiphon loop if overboard
discharging, the lift to the top of the holding tank is much less).
So, maybe it is possible to change to a top entry on the holding tank or, more simply, rearrange the pipework so that the pipe to the holding tank rises above the level of the tank top at some point between the toilet and the tank and so can only drain back what is on the toilet side of the rise.