All manual toilets should have a complete rebuild kit installed at least every 5-6 years...a new joker valve annually. You wouldn't replace just one spark plug at a time in your car's engine...it makes no more sense to replace just one part at a time in a marine toilet. All the parts in a manual toilet pump wear with use, wear faster if the pump isn't kept VERY well lubricated. If one part in it has worn enough to fail, the rest have to be seriously worn...and the worse they're worn, the more impact that has on the efficiency and performance of the pump. Replacing parts one at a time means also mean you have to take the pump apart each time to do it...and you may only be guessing at which part--or how many parts--it needs, so even though the toilet may be working again, it won't be anywhere near factory specs. So if the toilet is at least 5 years old and/or you don't know when or even IF it's ever been rebuilt, as long as you have to take the pump apart, bite the bullet and do the WHOLE job. You'll be amazed at how much better your toilet will work than it will with only one new part and the rest worn out.
As for WHY the joker valve should be replaced annually...most boat owner think it has only one function: to block backflow. They don't know that it's the single MOST important part in a manual toilet pump because they don't actually know how a manual toilet 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, making the toilet less and less efficient till finally it can't move anything out of the bowl any more, and you'll be here trying to find out why and usually getting the wrong answer.
