Sounds like the pump seals have failed.
In all piston/cylinder toilet pumps, there are rubber seals around the piston rod that are supposed to fit tightly against the cylinder wall, creating the "vacuum" needed for the pump to pull flush water in and push bowl contents out. Seals--and all the other rubber parts--wear out. They dry out and become brittle between weekend uses, and unless replaced fairly regularly they can crack and crumble. However, the seals aren't the only thing that wears...their friction against the cylinder wall wears it too--and not evenly...dry brittle seals can score the wall...and there comes a time when rebuilding can't return a pump to original specs.. On most toilets, that happens after about 7-8 years. Since--for some reason I'll never understand--99% of boat owners would rather take a toilet apart when something fails and the bowl is full than rebuild as preventive maintenance when it's clean and dry, odds are that your toilet hasn't been rebuilt in years, if ever. It's 16 years old, average price of a rebuild kit--which may not solve your problem--is about $50...and it's a whole lot easier to replace than rebuild. That's what I'd do. You aren't likely to find a toilet--even the same brand--that will fit a 16 year old toilet's bolt patterns in the floor...manufacturers keep improving and/or redesigning for more contemporary styling. So expect to do a little sole refinishing no matter what you buy, and base your purchase decision on which toilet you like the best, and fits in your head. For under $150 you can replace with several comparable piston/cylinder pump toilets. However, for a little more, the Raritan Cricket is a diaphragm pump toilet that I recommend highly...diaphragm pumps are far more efficient and there aren't any moving parts to fail in a diaphragm, nor any of those pesky pump shaft seals. Check it out on the Raritan website at www.raritaneng.com.
