Hydraulic system of construction machine

A hydraulic system of a construction machine, comprising:

• a slewing motor;
• a mechanical brake that is, when supplied with pressurized oil, switched from a brake-applied state, in which the mechanical brake prevents rotation of an output shaft of the slewing motor, to a brake-released state, in which the mechanical brake allows the rotation of the output shaft;
• a slewing control valve interposed between a main pump and the slewing motor, the slewing control valve including a first pilot port for a first slewing operation and a second pilot port for a second slewing operation;
• a first solenoid proportional valve connected to the first pilot port by a first pilot line;
• a second solenoid proportional valve connected to the second pilot port by a second pilot line;
• an auxiliary pump connected to the first solenoid proportional valve and the second solenoid proportional valve by a primary pressure line;
• a switching valve interposed between the auxiliary pump and the mechanical brake, the switching valve including a pilot port that is connected to the first pilot line by a switching pilot line, the switching valve switching from a closed position to an open position when a pilot pressure led to the pilot port becomes higher than or equal to a setting value;
• a slewing operation device that includes a lever and outputs a first slewing electrical signal when receiving the first slewing operation, the first slewing electrical signal corresponding to an operating amount of the first slewing operation, and outputs a second slewing electrical signal when receiving the second slewing operation, the second slewing electrical signal corresponding to an operating amount of the second slewing operation;
• and
• a controller that controls the first solenoid proportional valve and the second solenoid proportional valve based on the first slewing electrical signal and the second slewing electrical signal, wherein
• both when the first slewing operation is performed and when the second slewing operation is performed, the controller controls the first solenoid proportional valve, such that the first solenoid proportional valve outputs a secondary pressure higher than or equal to the setting value.