Abstract: A well service pump system supplies high pressure working fluid to a well. The pump system includes a closed-loop hydraulic circuit for actuating a plurality of working pump assemblies. The pump system is powered by a motor, which transfers mechanical energy to a plurality of pumps, which, in turn, provide hydraulic fluid to operate hydraulic ram cylinders, and thereby operate the working pump assemblies. Each of the polished rods of the hydraulic ram cylinders is connected axially to a plunger rod end of the working fluid end cylinder to operate the working pump assembly.