When you replace the Joker Valve, make sure to change the "Base Valve Gasket" also. This will prevent spill back into the bowl.
See diagram attached.
Always pump a gallon or more of fresh water before replacing, so the fresh water is the only thing that leaks
back into the bowl. The exit pipe goes up three feet, so you have to fill it with fresh water first.
To pro-long the time between changing the Joker and Base valves.
1) keep the intake valve closed.
2) Leave a cup in the sink and when flushing
just use a cup or more of fresh water from the sink. The problem is that the urine takes the salt
out of the ocean water and creates calcuim deposits on the rubber valves and then they can not seal shut.
3) The Joker valve can be cleaned and stored with a rubber band holding it in the closed position for a month and than can be reused again.
4) The Base valve usually has to be replaced every time