second interior cylindrical portion 245 in turn merges with an interior cylindrical portion 247 of the transverse bore 237 the guard 235 is sealingly fixed to the pad block exterior surface by bolts and serves to protect the sealing pad 197 the guard pad 235 has a central cavity 249 which receives the inner end portion of the piston shaft 229 the lower backup pad assembly 193 is like the upper backup pad assembly 191 except that its seal plug 251 does not incorporate circumferential groove 241 and consequently does not provide the associated fluid passage the equalizer valve assembly 195 comprises a piston 253 a seal ring 255 a retainer plug 257 and a bias spring 259 the pad block 39 is provided a bore 261 for receiving the equalizer valve assembly 195 the piston 253 matingly and sealingly engages adjacent its inner end a portion 263 of the pad block bore 261 and adjacent its outer end a central bore 265 of the seal ring 255 the inner end of the piston is exposed to a hydraulic fluid flow passage while the outer end is exposed to well bore fluid the retainer plug 257 threadedly engages the outer end portion of the pad block bore 261 to hold the seal ring 255 in place within a portion of the pad block bore 261 the bias spring 259 bears at one end on the seal ring 255 and at the other end on a shoulder on the piston 253 so as to urge the piston inwardly for a purpose to be hereinafter explained the lower tool section 19 with the exception of the pivot assembly.