a line 51 establishes parallel connection of the respective pilots of check valves 42 43 and the circumstance of sufficient hydraulic pressure in a control line 52 is operative to dislodge both check valves 42 43 from their normally closed condition this line 52 control connection additionally includes a solenoid operated valve 53 which is normally positioned to discharge pressure fluid in line 52 to sump but which is solenoid actuable to enable pressure fluid in either of the integrator port lines 46 47 42 39 43 39 to pass via line 52 for concurrent pilot driven opening of both check valves 42 43 there being isolation check valves 54 55 connected back to back at 56 to valve 53 to assure integrity of the described pilot operating connection 52 two further check valves 56 57 in separate lines 58 59 of connection from sump 33 39 to the respective port connections 46 47 of the power integrator are operative to assure an initial supply of hydraulic fluid to the power integrator no matter what the initial direction of drive from motor 49 specifically each of the check valves 56 57 is oriented to check or block any flow in the direction of sump 33 39 a brief operating description may now be given for the circuit of fig1 which is to be understood as an illustration of a first mode of use namely involving a variable flow power integrator 45 in combination with a reversible bidirectionally operable electric motor 49 as the prime mover initially one may assume a filled system wherein the pistons of cylinders 14 20 are locked at a particular level.