High pressure hydraulic systems

A high pressure hydraulic system comprising a source of high pressure fluid, a slave unit and a control valve assembly, fluid from said source being supplied to said slave unit through said control valve assembly which is operative to maintain said slave unit in a desired condition, said control valve assembly having first and second ports, said first port communicating with said source and said second port communicating with said slave unit, a valve member movable between a first closed position in which said valve member closes said first port, a second closed position in which said valve member closes said second port, and an open position in which communication between said first and second ports is permitted, movement of said valve member being controlled by forces produced by a first means comprising a solenoid assembly associated with each port, by a second means comprising means for applying a pressure differential across said valve member and by a third means comprising means for causing fluid flow past said valve member, said forces produced by energisation of said solenoid assemblies being of predetermined values, such that when a solenoid assembly of said first means is energized and said second means produces a force which opposes and exceeds the predetermined value of the solenoid force, said first means is unable to move said valve member, and said control valve assembly also having an elastomeric sealing means to provide a substantially leakproof seal at least when said valve member closes said first port.