A pressure control valve for a construction machine is disclosed, which prevents self-weight descent of a working device due to leakage of hydraulic fluid in a neutral position of the working device. The pressure control valve for a construction machine includes a holding valve block having a pilot signal pressure port and a drain flow path formed thereon; a holding poppet having a back pressure chamber formed therein to intercept leakage of hydraulic fluid from the hydraulic actuator to the control valve in a neutral state; a first elastic member pressing the holding poppet and elastically biasing an intercepted path between the hydraulic actuator and the control valve; a piston sliding in the holding valve block when pilot signal pressure is input through the pilot signal pressure port; a pilot poppet shifted to drain the hydraulic fluid in the back pressure chamber toward a drain flow path of the control valve in response to movement of the piston; a second elastic member pressing the pilot poppet and elastically biasing an intercepted path between the back pressure chamber of the holding poppet and the drain flow path of the control valve; and an auxiliary drain path draining the leaking hydraulic fluid through a drain path of the holding valve block if the leakage of the hydraulic fluid occurs on a check valve that opens or closes the path between the back pressure chamber of the holding poppet and the drain flow path of the control valve.

A fluid pressure control device includes a load holding mechanism that is configured to hold the load pressure of the load side pressure chamber. The load holding mechanism includes a switching valve having a communication passage configured to be blocked from the third pressure chamber by the second land section in a case where the spool is closed, and providing communicate between the second supply port and the discharge port in accordance with the movement of the spool in the valve opening direction. In a case where the spool is moved in the valve opening direction, at the same time when or after the second supply port communicates with the discharge port via the communication passage, the first land section is brought into sliding contact with the annular projecting section and the first supply port and the discharge port are blocked from each other.

An apparatus includes a hydraulic circuit that is configured to selectively open fluid communication between one portion of the hydraulic circuit and another portion of the hydraulic circuit, such as a reservoir, based on the flow of the hydraulic fluid in the one portion. When the flow of hydraulic fluid exceeds a selected threshold in the one portion of the hydraulic circuit, the flow of fluid urges the opening of a hydraulic component of the hydraulic circuit to allow fluid communication between the one portion and the reservoir to discharge fluid from the one portion.

Hydraulic control circuits, couplers with hydraulic control circuits, and methods for controlling couplers that include a safety knuckle for retaining a first pin and a tongue for retaining a second pin, the safety knuckle automatically resetting upon removal of the safety pin.

Hydraulic jack assemblies are disclosed herein. The hydraulic jack assemblies may include a hydraulic unit base that includes a plurality of pump pistons to output hydraulic fluids with different characteristics that can be selectively used based on load conditions.