Harv, (La Crosse, WI)
But the most common cause is simply that paper and waste have accumulated inside the slide valve mechanism on Thetford’s Aqua Magic units. On an older SeaLand toilet, the typical cause is a worn seal or a clamp ring that is too loose. Yours is likely to be one of these two brands.
If you have an Aqua Magic, fashion an L-shaped hook out of a coat hanger or a bent screwdriver that will reach into the groove to scrape out the residue. Turn off the water supply, depress the flushing pedal and carefully remove any paper jammed into the groove just below the rubber seal. There is a tool you can purchase from Thetford just for this task if you wish. Take special care not to damage that seal. In severe cases, the toilet will have to be disassembled and the flushing mechanism taken completely apart, then cleaned, lubed and reassembled.
To rectify the situation on the SeaLand toilet, simply tighten the clamp ring. In some cases, a complete seal kit may have to be installed if water still continues to seep past the seal and into the holding tank.