Hydraulic circuit system

A hydraulic circuit system comprising a hydraulic pump (1), a plurality of hydraulic actuators (3-1, 3-2) driven by a hydraulic fluid delivered from said hydraulic pump, a plurality of control valves (4-1, 4-2) disposed between said hydraulic pump and said plurality of actuators, a signal transmitting hydraulic line (9) to which a signal pressure based on a maximum load pressure among said plurality of hydraulic actuators is introduced, and pump control means (2) for controlling a delivery pressure of said hydraulic pump to be held higher than said signal pressure by a predetermined value,

• said plurality of control valves comprising respectively main valves (4a-1, 4a-2) including meter-in variable throttles (M/I) for controlling flow rates of the hydraulic fluid supplied to said hydraulic actuators, and
• flow distribution valves (5-1, 5-2) disposed between said meter-in variable throttles and said actuators, each of said flow distribution valves including a valve body (50) which has one end positioned on the inlet side (5a) of said flow distribution valve connected to said meter-in variable throttle and the other end positioned in a control chamber (70), said valve body being moved through a stroke depending on balance between a pressure in said control chamber and a pressure in said inlet side to control the pressure in said inlet side, thereby controlling a differential pressure across said meter-in variable throttle, wherein:
• said hydraulic circuit system further comprises a load-pressure detecting hydraulic line (20, 32, 31-1, 31-2;
• 7a, 8-1, 8-2, 10-1, 10-2, 7b) provided in each of said plurality of control valves (4-1, 4-2), said load-pressure detecting hydraulic line including a first hydraulic line (20, 32;
• 7a, 8-1, 8-2, 10-1, 10-2) with a check valve function, which is branched from a point between said meter-in variable throttle (M/I) and said hydraulic actuator (3-1, 3-2) for detecting a pressure at the branched point, and is connected to said control chamber (70) of said flow distribution valve (5-1, 5-2), and a second hydraulic line (31-1, 31-2, 7b) for connecting said control chamber to said signal transmitting hydraulic line (9), said first hydraulic line with the check valve function including a valve body passage (20), which is formed in a valve body (50) of said flow distribution valve (5-1, 5-2) and has one end being opened to one of the inlet side (5a) and the outlet side (5b) of said flow distribution valve and the other end being opened to an outer periphery of said valve body, and a lap portion (32) located between the other end (20a) of said valve body passage and said control chamber (70) and making the other end of said valve body passage opened to said control chamber when the valve body of said flow distribution valve is moved through a stroke of a predetermined distance in the valve opening direction;
• a selector valve (11) provided in said second hydraulic line (31-1) of said load-pressure detecting hydraulic line in a first particular control valve (4-1) of said plurality of control valves;
• and
• a third hydraulic line (35) connected to the outlet side (5b) of said flow distribution valve in a second particular control valve (4-2) of said plurality of control valves,
• said selector valve (11) having a first position (I) at which a portion of said second hydraulic line (31-1) on the side of said control chamber is connected to only said signal transmitting hydraulic line (9), and a second position (II) at which the portion of said second hydraulic line (31-1) on the side of said control chamber is connected to both said signal transmitting hydraulic line (9) and said third hydraulic line (35).