Most people--apparently including y'all--don't have a clue what a joker valve's function really is...you think that the only thing the joker valve does is acts as a check valve to stop backflow from returning to the toilet or odor from the tank from escaping through the toilet. But that's not a joker valve's most important function...in fact, the joker valve is THE single most important replaceable part in a manual toilet.
Here’s how the discharge half of the pump works: 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 contents of the toilet bowl 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 generated when you pump it. And as it becomes more worn, less and less vacuum is created, till finally the bowl contents simply move up and down a bit, but don't go anywhere.
You prob'ly won't notice the loss of efficiency at first because it's so gradual...same as we don't see that we've gotten a little older than we were yesterday when we look in the mirror each morning. 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 to the tank or all the way out the thru-hull....IF they're getting there at all any more. Sometimes the flapper valve needs to be replaced too, which is one of the reasons why toilets should also be rebuilt at least every 5-6 years as PREVENTIVE maintenance.
The above is an excerpt from my latest book. If you have it, you might consider reading it! If you don't have it, get it (see link in my signature)...you'll find all kinds of useful sanitation system info in it, including the best way prevent runback while also increasing the number of flushes your tank can hold by up to 50% by reducing the amount of flush water going into it. And also why increasing air exchange via the tank vent is the key to eliminating odor out the vent when you flush.
