rod 12 are defined forwardly by nut 38 contacting end cap 17 and rearwardly by rear face 26 contacting gland 18 from its rear face 26 piston head 21 has a reduced diameter for a portion of its axial length which forms a rear ledge 39 and a ledge cylindrical surface 40 valve passageway 33 has a main bore 42 with a diameter x and at its rear end a counterbore 43 with a diameter y counter bore 43 opens to both rear face 26 and rear ledge 39 the bottom 44 of counter bore 43 is recessed forwardly of rear ledge 39 a distance m and forwardly of rear face 26 a distanced n referring to fig1 3 valve assembly 22 includes a valve member 48 and the valve passageway 33 valve member 48 includes a central shank 50 a forward head 51 and a rearward head 52 shank 50 has a generally round cross section body 55 with a diameter t just slightly smaller in one embodiment about 0 005 inches in diameter than the diameter x of valve passageway 33 so that shank 50 can freely slide therewithin at its rearward end 53 shank 50 is rigidly connected with rearward head 52 the junction 54 between shank 50 and rearward head 52 forms not an abrupt 90 degree corner but is instead slightly radiused which contributes to a better seal between valve member 48 and piston head 21 the forward end 57 of shank 50 is reduced in diameter and is threaded and the junction 58 between the cylindrical body 55 and the threaded portion 57 forms a ledge 59 forward head 51 is.