Hydraulic drive system

A hydraulic drive system comprising:

• a plurality of control valves disposed on a circulation line extending from a main pump to a tank;
• a plurality of operation devices with which to operate the plurality of control valves, each operation device of the plurality of operation devices including an operating lever and outputting an electrical signal in accordance with an inclination angle of the operating lever;
• a plurality of solenoid proportional valves that output pilot pressures to the plurality of control valves;
• a controller that controls each solenoid proportional valve of the plurality of solenoid proportional valves based on the electrical signal outputted from a corresponding one of the plurality of operation devices;
• a primary pressure line that leads hydraulic oil from an auxiliary pump to the plurality of solenoid proportional valves;
• a solenoid switching valve provided on the primary pressure line;
• a movement detection line that extends from the auxiliary pump to the tank through a plurality of movement detection target control valves among the plurality of control valves, the movement detection line being blocked when any one of the plurality of movement detection target control valves has moved;
• and
• a movement detection pressure sensor provided on the movement detection line, wherein
• the controller controls the solenoid switching valve while all of the plurality of operation devices are outputting electrical signals indicating that their respective operating levers are in neutral, such that:
• the solenoid switching valve opens the primary pressure line if a measurement value of the pressure sensor is less than a threshold;
• and
• the solenoid switching valve blocks the primary pressure line if the measurement value of the pressure sensor is greater than the threshold.