A couple of questions
What make/model toilet...and how old? It may be that the pump housing is just so worn and scored from salt left to dry in it that it's beyond help. When you rebuilt, did you slather everything with either SeaLube or SuperLube? They're both really thick lubricants especially for the head, and should last a season unless the housing is too far gone. Do you pump some fresh water through the toilet before closing up the boat to flush the salt out of the pump? When it's left to dry in the pump, it scratches the pump housing...A rough surface not only destroy seals faster, but isn't gonna pump as easily as a smooth surface. If you think you need to carry a lubricant by the case, I suspect you may be operating on the theory "if a little is good, a lot is better."...but a cupful will wash out as fast as a tablespoon. If it works fine for a couple of weeks after you add some lubricant, why not just make it a habit to put a LITTLE in the toilet once a week? Pump some fresh water through it, followed by no more than a tablespoon of head lube (what ARE you using btw?) at the end of each weekend.
