Apparatus for transmitting pressure from a hydraulic fluid to a slurry material includes a housing having a piston chamber defined therein surrounded by a piston chamber wall. The housing also contains an inlet port and an outlet port, through which ports slurry material enters and leaves the piston chamber. A piston is slidably disposed in the chamber so that sliding movement of the piston produces pressurization of the slurry material and movement thereof through the outlet port. A hydraulic fluid channel is disposed in the housing so that pressurized hydraulic fluid therein produces a force on the piston which causes sliding movement thereof, so as to produce a pressure in the slurry material which is proportional to the pressure in the hydraulic fluid. Preferably, the hydraulic fluid channel is further disposed in flow communication with the clearance gap between the outer surface of the piston and the corresponding surface of the piston chamber wall, so that at least a portion of the pressurized hydraulic fluid passes through the clearance gap and flushes solid particles therefrom. In a preferred embodiment, the apparatus further includes means for counterbalancing the force produced by the slurry material on the interior surface of the piston chamber wall, with an oppositely directed force on the oposite surface of the piston chamber wall.