hello, we're having a problem of boom lowering on the Hitachi 870-3 excavator. when the oil is a bit cool, the machine operates normally but after loading for about 10 minutes it starts to fail to lower the boom, it jerks. it only responds or lower the boom if you try to raise the boom and then immediately lower the boom. we have replaced, the Joystick, control valve block, relief valves and swapped spools on the main valve block with another similar machine. Also changed the four solenoid block but no change. we are now clueless as to what could cause these problems. unfortunately i do not have a trouble shooting guide , but only a schematic for hydraulics and electrical of i am failing to solve this problem.
remove the boom reduced leakage valve and try it