When water is supplied to the pump outlets and pipelines, air accumulations in the system, especially in the pump outlets, narrow the water passage area and cause a decrease in capacity. Double sphere suction cups are used to avoid this drawback and where high efficiency is required. When water is pumped into the system, the air that is formed is sucked into the suction cup before the water and discharged through the holes in the two inner side covers. The suction cup is then filled with water, lifting the balls up and sealing the holes.