Yesterday I did a major overhaul to the Thetford Headmate toilet on my boat. I videotaped the process and when I get some spare time I'll edit it and put it on YouTube, with a link placed here. The main symptom was that no matter how much pumping you did, the liquid sort of stayed in place. It never exited the toilet to make its way to the holding tank. Because the toilet was not flushing, I assumed (mistakenly) that all it needed was a new joker valve. After checking around various parts sources, I begrudgingly purchased the complete Thetford repair kit # 37057 because I could find no source for the joker valve alone. The joker valve only comes as part of the kit, which was $83 at West Marine. I also purchased a Jabsco joker valve, $12, to see if it might substitute for the Thetford.

There's more to this story which the video will relate, but here is what I found:
1) The main problem was that BOTH the flapper valve AND the joker valve were warped with age, and this distortion prevented either from sealing properly. Their proper sealing is what gives the assembly it's one-way pumping action.   
2) I also found that the pump would not move flushing water to the rim of the bowl when the switch was in the Flush (ie, Wet) position. This fix required some of the other parts that were in the kit, so maybe there was some wisdom in Thetford's kit packaging.
3) Importantly, I discovered that the Jabsco joker valve DID substitute for the Thetford. In fact, I reassembled the unit with the Jabsco joker valve in place and I am saving the Thetford as a spare.