A sealing device for a high pressure hydraulic piston mechanism in which the gap between the cylinder in which the piston is slideable to pressurize delivery fluid therein is sealed by sealing fluid drawn from a separate supply to that of the delivery fluid, delivery means being associated with the sealing fluid supply and being operable in response to pressurization of the delivery fluid to deliver sealing fluid to said gap at a pressure sufficient to form a seal therein, the sealing fluid of said seal being in direct contact with the pressurized delivery fluid and the piston being movable in at least one hydrostatic bearing supplied with sealing fluid in parallel to the supply thereof to said gap between the piston and the cylinder.