piston 40 comprises a cylindrical body 51a having an exterior annular shoulder 52 thereon as shown in fig2 of the drawings the shoulder 52 engages the lower shoulder 47a in the cap 47 to limit the downward travel of the piston 40 relative to the cap 47 further the cylindrical piston 40 is provided with a plurality of circumferentially spaced bores 60 which receive the pins 62 that are threaded into the piston header 80 as shown the lower end 40b of the piston 40 engages the upper end plate 56 of the spring cartridge designated generally s such spring cartridge includes three concentrically mounted coil springs s 1 s 2 and s 3 which are positioned between the upper end plate 56 and the lower end plate 57 such spring cartridge acts as a return spring to return the piston 40 to the upper position shown in fig1 the piston header 80 is a cylindrical cap which is provided with a lower flange 81 having a plurality of bolts or screws 82 for connecting the header 80 to the piston cap 47 also the piston head includes an annular groove 84 which extends around the header near the upper end for receiving the connecting plate 88 which is connected by means of yoke 89 to the override piston cylinder 90 as shown the cap 80 includes hydraulic fluid passage 85 which is connected to conduit 85a for introducing hydraulic fluid into the bore 50 in the cap 47 for moving the piston 40 axially downward of the valve actuator a also as shown in fig1 a fluid discharge conduit 86 is also provided in the.