I have a Sloan-Flushmate installed in 1999 time frame. Is it known that the tanks can ultimately begin decomposing, rotting or corroding from the inside such that no amount of flushing will clear small "crumbs" of small gravely particles from the bowl bottom?. It will appear to fully clear during a flush, but over seconds reappears as if fresh material is in the tank. The flushing force is as strong as ever and consistently clears out all waste and seems to remove the last batch of "stuff" on each flush. Our tank is black plastic with stainless end caps. Over the next few minutes after a flush a fine tan/brown dust of sort will settle (under water) on the bowl surface. I've checked the inlet line and there is no debris coming in that way. I hoped it was simply a rotting washer or packing gasket in the inlet valve. Nope. We have 3 of these units, all the same age and this is the only one doing this. I'm about to order a new toilet. Any advice welcomed, thanks.