A hydraulically actuatable piston which is subjected to a constant bias (e.g. gravity) in a direction opposite to that in which it is movable under hydraulic pressure is provided with means for accurately maintaining a stationary piston positioning under hydrostatic pressure despite leakage past the piston by a metering valve actuatable by differential movement between the piston and cylinder, fluid being constantly supplied to the underside of the piston and metered past the valve.