Abstract: A working head is adapted to impart reciprocal motion under a predetermined amount of force to a plunger through a working and return cycle and at a controllable rate of speed. A fluid circuit includes delivery and return lines extending between a source of fluid under pressure and the working head, and pilot control means associated with the fluid pressure source is operative to regulate the working and return cycle. The working head has a plurality of annular flexible casings of toroidal configuration with opposite sidewalls terminating in mutually opposed, inner bead-like edges and interconnecting means extending between the edges of adjacent casings serve to interconnect the casings in sealed, coaxially spaced relation to one another. Closures at opposite ends of the plurality of casings define a common sealed chamber therebetween. A support member fixes one end of the chamber against movement and permits axial expansion and contraction of the opposite end of the chamber with respect to the fixed end.