A pumping unit (20) includes a hopper assembly (24) for holding material to be pumped, a pair of identical pumping assemblies (26, 28), and a pumped material outlet (30). The assemblies (26, 28) include directional or spool valves (54) having rotatable spools (76), tubular pumping chambers (56), and pistons (122) within the chambers (56). The pistons (122) include concave operating faces (134), which are complemental and mate with the outer surfaces of the spools (76). Operation of the unit (20) creates successive charges of pumped materials having a minimum of disruptions, such as tearing or smearing.