Valve with improved piston with elastomer O-ring

A piston type valve including a first piston moving in a cylindrical seat, wherein said first piston is fastened to an operating rod extending along an axis of said cylindrical seat, said first piston having a perimeter which carries a first elastomer O-ring for providing a seal between said first piston and said cylindrical seat, said first piston including first and second piston members fixed axially together by clamping means, said first piston member having a first transverse surface and a first annular groove of substantially circular arc shaped cross section, said first annular groove being at a distance from said perimeter of said first piston and being connected to said first transverse surface by an inner edge and an outer edge each having a substantially rounded shape, said second piston member having a second transverse surface and a second annular groove of substantially circular arc shaped cross section, said second annular groove being at a distance from said perimeter of said first piston and being connected to said second transverse surface by an inner edge and an outer edge each having a substantially rounded shape, said first transverse surface facing said second transverse surface and said first annular groove being open toward and in alignment with said second annular groove, said first elastomer O-ring is housed between said first and second annular grooves and clamped between said first and second annular grooves by said clamping means, and an empty space between the first and second transverse surfaces into which empty space said elastomer O-ring can deform after fitting said O-ring and clamping said first and second piston members together.