Use of optimized piston member for generating peak liquid pressure. A one-step hydraulic blow molding system and method for forming a preform and a liquid filled container from the preform. The machine and method include a sealing mechanism that forms a seal primarily utilizing compressive forces in an axial direction to prevent the leakage of liquid blow medium. The sealing mechanism includes an axial end face of a forming head and an upper axial surface of a neck ring.