Abstract: Problem: In a hydraulic system in which a manual line relief valve that adjusts the relief setting pressure is arranged in the line relief oil path formed by branching from the actuator line from the control valve to the hydraulic actuator, the relief setting pressure adjustment operation of the line relief valve can be performed independently with good accuracy. Solution: A controller 20 for controlling a perform of a relief setting pressure adjustment operation of a line relief valve 25 is provided with a pump flow control means that controls the pump flow rate during the relief setting pressure adjustment operation, an instruction command output means 54 that instructs a perform of the relief setting pressure adjustment operation by a manual operation, and a notification command output means 55 that notifies that the pump pressure detected by a pressure sensor 31 during the relief setting pressure adjustment operation is within a predetermined range of the target relief setting pressure.

Abstract: Problem: In a hydraulic excavator equipped with a swivel parking brake, the number of components is reduced and the circuit is simplified while the swivel parking brake is released during the operation of the working actuator as well as during the swivel. Solution: A common electromagnetic proportional valve that outputs a pilot pressure to the straight travel valve is shared as a pilot pressure output electromagnetic valve that outputs a pilot pressure to the brake switch valve, and a pilot pressure (second set pressure) necessary for switching the brake switch valve is set to a lower pressure than the pilot pressure (first set pressure) necessary for switching the traveling straight-going valve.

Abstract: To provide a relief pressure control device in which a relief pressure can be set easily, and further power consumption is suppressed. A relief pressure control device is equipped with a relief valve in which a setting pressure is made variable as a function of an energization quantity, and a controller configured to set at least one relief pressure of a fluid pressure circuit by controlling the energization quantity of the relief valve. The controller energizes the relief valve, when the pump pressure exceeds a predetermined threshold pressure lower than the setting pressure of the relief valve that is in its non-energized state.