| 摘要 |
To permit smooth performance of both of an operation requiring a high pressure and an operation desired to produce a pressure at a suppressed level, a shuttle block 50 positioned between pilot operating units 35 - 37 and flow control valves 5 - 15 and pump regulators 28 a, 28 b is arranged in association with shuttle valves 61 - 63,65 - 75 , which select maximum pressures of groups of operation signal pressures produced by the pilot operating units 35 - 37 , respectively, and at least one of the plural groups of operation signals, and is constructed to include hydraulic selector valves 81,82 and a boom-lowering, hydraulic selector valve 83 . The hydraulic selector valves 81,82 are operated based on the maximum pressures to produce control signal pressures from a pressure of a pilot pump 2 . The boom-lowering, hydraulic selector valve 83 is operated based on an operation signal pressure Dd relating to a single boom-lowering operation among the operation signal pressures produced by the pilot operating units 35 - 37 to produce a boom-lowering, control signal pressure from the pressure of the pilot pump 2.
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