Hydraulic drive system and directional control valve

A hydraulic drive system for a construction machine comprising hydraulic pressure supply means;

• a plurality of actuators driven by a hydraulic fluid supplied from said hydraulic pressure supply means;
• and a plurality of directional control valves respectively disposed between said hydraulic pressure supply means and said plurality of actuators, and each comprising a pump port, a pressure chamber capable of communicating with said pump port, a feeder passage capable of communicating with said pressure chamber, actuator ports capable of communicating with said feeder passage, a reservoir port capable of communicating with said actuator ports, first meter-in variable restrictors disposed between said pump port and said pressure chamber, and a pressure compensating valve disposed between said pressure chamber and said feeder passage and having a pair of opposite ends, one of which is subjected to a pressure in said pressure chamber and the other of which is subjected to a maximum load pressure among said plurality of actuators, said hydraulic pressure supply means having a hydraulic pump and pump flow control means for controlling a delivery rate of said hydraulic pump so that a delivery pressure of said hydraulic pump is held higher by a predetermined value than the maximum pressure obtained, as a load sensing pressure, from load pressures of said plurality of actuators, wherein:
• at least one of said plurality of directional control valves has a bleed passage for communicating between said feeder passage and said reservoir port, and second variable restrictors disposed in said bleed passage and moved in conjunction with said first meter-in variable restrictors and being kept open after displacement of the corresponding directional control valve from a substantially neutral position and after said first meter-in variable restrictors are opened.