that passes through a plug 152 in the open end of the bore at its outer end stem 150 is fitted with a button 154 the closed end of bore 146 is in fluid communication with the closed end of cylinder 68 through first end port 156 in manifold 36 and passage 158 in housing 12 the end of bore 146 next to plug 152 is in fluid communication with the rod side of piston 70 through second end port 162 in manifold 36 and passage 164 in housing 12 a center port 166 in bore 146 communicates through passage 168 in housing 12 with passage 164 to chamber 160 on the rod side of plunger 70 a vent port 170 connects cavity 187 defined as the annular space between the reduced diameter portion 178 of shuttle 148 and the wall of bore 146 with waste or outlet port 114 a pressure port 172 connects cavity 189 defined as the annular space between the reduced diameter portion 176 of shuttle 148 and the wall of bore 146 with the passage 130 which is in fluid communication with the concentrate side of element 54 by means of passage 132 relief valve 128 is disposed between passage 132 and opening 114 shuttle 148 has a center land 174 the length of which is slightly more than three times the width of center port 166 shuttle 148 has reduced diameter portions 176 and 178 on opposite sides of center land 174 reduced diameter portion 176 extends between center land 174 and a packing land 180 at the bottom end of shuttle 148 reduced diameter portion 178 extends between center land 174 and packing land 182 at the top end of shuttle 148 for internal.