seats the spring 100 between it and the bypass valve 95 the cap also fixes the wiper seal 110 between it and an outer ledge 180 in the piston body the piston 50 moves upwardly and downwardly within the valve body 45 and within the piston cap 55 which is conventionally fixed for easy access within the valve body 45 an area 185 for holding fluid is defined in the piston cap 55 above the piston 50 the wiper seal 110 extends beyond the edges of the valve to form an interference fit with an interior wall 190 of the guide see also fig4 as will be discussed herein referring to fig3 the piston body 70 is shown the piston body has a neck 200 a body portion 205 having a larger perimeter than the neck and a shoulder 210 having a rounded portion 220 and a larger perimeter than the neck the body portion has a taper 225 therein that slopes inwardly towards the neck 200 in the embodiment shown herein the taper is disposed at an angle of approximately 20 u00b0 relative to the shoulder and has a length of approximately 0 168 inches the rounded portion 220 of the shoulder 210 has a radius of approximately 0 04 inches other combinations and permutations of radius angle and length may be used in other valves if they provide the benefits of this invention referring to fig4 and 4 a the piston cap 55 is shown having in the embodiment shown a groove 230 having a depth of approximately 0 006 inches and a u00f8 of about 0 040 inches disposed in the inner wall 190 the groove extends from a bottom 235 of the valve guide 55 to.