I have got an intex 16 foot above ground pool and each time i have been doing a backwash sand comes out thru the exit point of the pump, it says you are supposed to turn pump on when doing backwash but if i do so the sand flies out thru the outlet and if i let it continue to go out then there would be no sand left in the housing. even just with it on backwash and the cap off with pump off sand still comes out. intex nz sent me a new pump which i assembled yesterday and the new one does the same thing.. has anyone else had this issue? the pump is a sand filter pump model SF90220 .