A molding system for hydraulic blow molding of a container from a preform. The molding includes a pressurization unit that is configured to receive liquid molding medium from a source; a fill head unit coupled to the pressurization unit to receive the liquid molding medium therefrom; a mold assembly defining a mold cavity in the shape of the container. During forming of the container, a nozzle piston is moved from a retracted position to an extending position where the nozzle piston is engaged to inject the molding medium into the preform, thereby forming and filling the container. The fill head unit defines a receiving chamber for the liquid molding medium that maintains a constant volume during movement of the nozzle piston between the retracted and extended positions.