US Patent 7937979 Gravity fill system with pressure check valve

A hydroform die includes a plurality of seal units to provide fluid to die cavities. The plurality of seal units each define a piston cylinder. A piston is located within the cylinder and is selectively moveable between an open and a closed position. Additionally, the piston at least partially defines a valve chamber located on a first side of the piston and a piston chamber located on a second side of the piston. A piston fluid passage is defined by the piston to fluidly connect the valve chamber and the piston chamber.