I have a 21" sand filter, 3/4 hp motor that I got used last year. I have another post in this forum trying to identify it. Anyhoo, for the last few days the water pressure on the return line has seemed quite weak. I would think it was a clog, but I have checked everywhere except in the sand itself and have found nothing. The pressure also does not seem high enough for a clog, only about 7psi. The odd thing is, I have great pressure when backwashing or dumping to waste, which leads me to believe the problem is after the pump. Any help would be appreciated.