There is a device by the pump where i can manipulate where the suction occurs. When I turn the dial to have the suction from the skimmer, water flows into the pump. When I turn the dial to suck from the main drain, not much water runs into the pump. Just to note most of the time a i have the suction sharing between skimmer and main drain. Due to water loss, water is starting to go under skimmer, so I decided to pull water from Main drain and saw no suction into the pump.