Abstract: A swimming pool installation including a skimmer device, a vacuum system, a pump and a valve module is disclosed. A seat divides the valve module into an upstream chamber coupled to the skimmer device and a downstream chamber coupled to the vacuum system and the pump. A piston having a seal is mounted in the upstream chamber for reciprocal movement in opposition to the seat and is biased away from the seat. A pawl is mounted to the piston for rotation and stops are mounted adjacent the pawl. The piston moves reciprocally in response to a cyclical application and removal of water movement there across caused by a cyclical operation of the pump, which causes the pawl to interact with the stops and cycle between a locked condition securing the seal away from the seat and an unlocked condition permitting the seal to move against the seat.