These valves are used to make two cylinders work in sequence; the second one starts when the setting is overcome. The check valve grants the free flow on the opposite direction. With these kind of valves the two cylinders pressures are added up, therefore the second cylinder pressure has to be limited. The external body is made of high resistance steel and externally protected from oxidations by zinc treatment. On request we can supply a zinc/nickel treatment to increase the corrosion resistance when the valve has poor protection and it is located in external environments.