The Tsunami hand help pump is a live well aerator pump. They push water, but will not suck it. As soon as the water level drops enough below the impeller, flow stops. I believe the integrated pumps are also aerator pumps. So this is also why some water is left in the sacs. However, one aid to get a more complete drain, is to use a quality check valve in the vent line. This prevents air from replacing the drained water and makes the sac "raisin" in on itself, forcing more water to the drain port.