I can pump 50 times but it seems that I can't get the final pint of so of flush water over the top of the discharge line and into the holding tank.
Are you pumping in the dry mode? If you aren't, you're bringing more water in at the same rate you're pumping it out. If you are, something's pushing back...most likely a blocked or partially blocked tank vent that's pressurizing the system, creating back pressure. That you're hearing waste fall into the tank does not rule out a vent blockage, especially if enough time has elapsed since the last use to allow any pressure to dissipate.
...I suppose that is normal or what else would be the need for the joker valve. I can't figure out what caused the backfilling other than the Joker valve being distorted.
If you left vinegar sitting in the bowl or didn't rinse behind it after about 30 minutes with about a quart of fresh water, it very well could be....which would also be the reason your toilet can't move the last of the water over the top of the loop. But even a brand new joker valve will only block slow seepage for very short time...only until enough flushes have gone through it to let even a little daylight show through the slit. The more you use the toilet, the wider the slit becomes and the less efficient your toilet becomes at moving waste to its destination.
You asked "what else could a joker valve be for?" But that's a joker valve's LEAST important function. It's actually is THE single most important replaceable part in a manual toilet.
Here’s why: On the upstroke of the piston, a vacuum is created in the area beneath the piston. This causes the joker valve to close tightly, and the flapper valve beneath the pump to open, allowing some of the bowl contents to be drawn into the bottom half of the pump. Then, on the down stroke of the piston, the flapper valve is slammed shut, and the effluent is forced out of the bottom of the pump, through the joker valve, and off down the line. But when the joker valve becomes worn and/or there's a buildup of sea water minerals on it, it can no longer seal tightly on the upstroke of the piston...less vacuum is created when you pump it. And as the joker valve becomes more worn, less and less vacuum... till finally the bowl contents simply move up and down a bit, but don't go anywhere. Sometimes the flapper valve needs to be replaced too, which is why toilets should also be rebuilt at least every 5-6 years (or a Jabsco pump replaced at least every two years) as PREVENTIVE MAINTENANCE
You prob'ly won't notice the loss of efficiency at first because it's so gradual...But I guarantee you that if it's been two years or longer since you replaced the joker valve, you need to pump the toilet at least 50% more times to move the bowl contents all the way to the tank or out the thru-hull....IF they're getting there at all any more.
I pump thru a bowl fresh water each time I use the boat to keep the system lubed and operational.
Water does not lubricate a pump. That requires grease. There are two ways to keep a toilet lubed....
1) the never-ending job of flushing a tablespoon of oil (any more won't last any longer and just is a waste of oil) every week or two...and btw, mineral oil is ok in an emergency (baby oil is mineral oil that smells nice), but it's a petroleum product that's not all that good for rubber.
2) Spend about 15 minutes once every year as part of spring recommissioning replacing the thick teflon or silicon grease that was in the pump when it left the factory. Complete instructions earlier in this thread.
Y'all really ought to check out the link in my signature...it goes to a comprehensive "marine toilets and holding tanks 101" manual that will help you prevent 99% of problems instead of having to solve 'em.
