I backwashed my pool today and they set it on rinse. I saw the sight glass fill with sand and then looked in my pool and saw a pile of sand, ~50 lbs of sand, under each return line. i moved it to waste so i could vacuum the sand and now my mltiport in stuck on waste. I think I usually use waste after backwash, but rinse should not cause sand to be dumped into pool should it? I have not done anything with my sand filter before having this problem. Is this possibly a problem with my spider gasket? My multiport looks really old. I just recently bought this house last year so I dont know much about the pool. I know the pool was built in 1976. Anyone have any ideas what could cause the sand to go into pool. I have an ingound pool, 27,500 gallons. The pump is pentair. I think the multiport is a hayward. The sand filter does not say anything on it, it a big stainless steel tank.