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1. A hydraulic control system in a working machine installed with an optional control circuit commonly used among multiple optional hydraulic actuators selectively mounted to the working machine; wherein the optional control circuit is installed with an optional flow control valve controlling a supply flow rate from a hydraulic pump to the optional hydraulic actuator, and an optional control valve controlling a discharge flow rate from the optional hydraulic actuator to an oil tank as well as changing over supply/discharge directions of hydraulic oil for the optional hydraulic actuator, so that the supply and discharge flow rates of the optional hydraulic actuator may be configured to be controlled independently of each other; wherein at an operating position of the optional control valve, a normal control region, where a discharge valve passage from the optional hydraulic actuator to the oil tank has an opening area preset out of normal control range, and a back pressure reduction region are provided where the discharge valve passage has a back pressure reduction opening whose opening area is larger than a maximum opening area in the normal control range; wherein the optional control circuit is installed with a controller, which, when the optional hydraulic actuator is a specific hydraulic actuator which is prespecified to need a back pressure reduction, positions the optional control valve in the back pressure reduction region during a running of the optional hydraulic actuator, and when the optional hydraulic actuator is not the specific hydraulic actuator, positions the optional control valve in the normal control region.
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