Hi. This has been going on for a couple of seasons now. After back washing and rinsing my sand filter I come back later to find clumps of very fine silt at the bottom of my above ground pool. There is enough of this silt that it doesn't look very inviting so vacuum it up before using the pool. I've just changed the sand in the filter and it still does it. Could this be a problem in the multiport valve or perhaps I'm doing something wrong? Can the multiport valve be rebuilt? Thanks.