flow into chamber b is shut off and a pressure of 10 bar maintained there consequently fluid pressure continues to rise in chamber a only causing an increasingly unbalanced force on piston 16 the valve piston 8 begins moving towards side b against the force of its centering spring which requires 30 bar pressure for full movement some fluid is being displaced by piston 8 into the tr line referring to fig4 the third step in extending the main piston occurs beginning at approximately 20 bar hydraulic pressure the continuous pressure overcomes the mechanical bias and moves the piston 20 of the position locking device so as to disengage the lock at the same time the stem of the b side of the valve piston 8 has entered the throttling bore of valve cartridge 4b and is in contact already with check valve ball 10b main piston 16 is still held hydraulically in its position by counterpressure building up in chamber b referring to fig5 the fourth step in extending the main piston begins at approximately 30 bar further pressure rise causes the valve piston 8 to open the check valve 10b and allows a restricted flow out of chamber b into the drained tr line piston 16 now extends at the speed determined by the pump delivery due to the throttling function of the stem of piston 8 in the discharge orifice of the valve cartridge 4b.