While doing some work on our toilet system, I noticed another possible failure mode that will cause the joker valve to fail to seal and let "stuff" flow back into the toilet. We had a thread that explored a lot of possible causes of this unpleasant problem a little while ago. This is one possible cause I think we missed on that round of discussions.
Hard "calcium" deposits build up in the discharge line from the toilet to the holding tank. It is possible for these deposits to accumulate in the fitting where the "tail" of the joker valves extends. The diameter of that fitting can shrink to the point that you install a perfect good new joker, but it is pinched in the fitting and held slightly open, causing the back flow that people have so much trouble with.
Moral of the story: Be sure that fitting is clean to its full diameter every time you change the joker valve.
Fort Lauderdale, FL