respectively the latter of these threaded portions mounts a trailing skip adjustment screw 173 which carries a slender rod 174 projecting inwardly toward the shoulder 170 in similar fashion the threaded portion 172 mounts a lead skip adjustment screw 175 which carries a slender rod 176 also projecting toward the shoulder 170 a ball valve 177 of slightly larger diameter than the reduced diameter portion 169 of the through bore is interposed between the inner adjacent ends of the rods 174 and 176 and is normally biased towards the shoulder or seat 170 by a helical coil spring 178 interposed between the ball valve 177 and the inner face of the trailing skip adjustment screw 173 as is shown in fig1 the inlet port 167 has direct communication with the larger diameter portion 168 of the through bore of the cylinder block 151 accordingly in view of the restriction afforded by the reduced diameter portion 169 of the through bore and the ball valve 177 the upstream actuator piston 153u is normally pressure activated prior to the downstream actuator piston 152d in the usual case the skip adjustment screws 173 and 175 are adjusted in such a fashion that the rod 176 of the downstream skip adjustment screw 175 holds the ball slightly unseated from the shoulder 170 the upstream adjustment screw 173 being set to define a gap between the adjacent ends of the rods 174 and 176 slightly larger than the diameter of the ball valve 177 with this arrangement very fine adjustments of the time delay between actuation of the upstream actuator.