a hydraulic pumping system is provided for delivering a compressible fluid material from a hopper to an outlet . a primary hydraulically driven pumping unit has a first interior volume for receiving the fluid material from the hopper . a secondary hydraulically driven pumping unit is interconnected with the primary hydraulically driven pumping unit and has a second interior volume less than the first interior volume for receiving fluid material from the primary hydraulically driven pumping unit . the primary and secondary hydraulically driven pumping units have hydraulically driven reciprocating piston units with unequal stroke lengths . the piston units are alternately reciprocated to fill the first interior volume of fluid material from the hopper , pump a portion of the first interior volume of fluid material into the second interior volume and the remainder of the first interior volume to the outlet , and pump the second interior volume of fluid material to the outlet while fluid material is being pulled into the first interior volume in a sequential and simultaneous manner which will produce substantially continuous delivery of fluid material to the outlet .