A hydraulic blow molding system for forming a container from a preform by injecting a liquid product into the preform. The system includes a seal pin located within the forming head and moveable between a closed position and an opened position. In the closed position a sealing portion of the seal pin is in sealing engagement with a portion of the forming head so that an exit orifice is closed and liquid product is prevented from being injected into the preform. In the opened position, the sealing portion is disengaged from the portion of the injection head so that the exit orifice is open and the liquid product can be injected into the preform.