One of the common reasons in such cases is a faulty PRV Valve (Pressure Reducing Valve), which is often overlooked.

It should be noted that in these excavators, although a pilot pump exists for the control circuits, the pressure required to actuate the PC valves and the servo pistons inside the main pump is supplied directly from the first stage of the main hydraulic pump.

In fact, the output pressure from the first stage of the pump is routed through an oil line to the PRV valve block, which is mounted as a small block on the Main Control Valve. Inside this block, the pressure is reduced and stabilized to approximately 35 bar (kg/cm²) by a regulator valve.

This regulated pressure is then sent back to the pump and is used to control the operation of the two PC valves in the main pump. This pressure plays a critical role in adjusting the swash plate angle, thereby controlling the pump’s output flow and pressure.

Additionally, this pressure acts as a reference pressure within the hydraulic control circuit and also contributes to the operation of the spool valves inside the main control valve.

It should be noted that the regulator valve inside the PRV is adjustable, allowing the servo line pressure to be set. The standard setting for this pressure is typically 35 ± 3 bar (kg/cm²).

Therefore, it can be concluded that the performance of the PRV valve is highly critical, and if it is faulty or improperly adjusted, the operation of the excavator’s attachments will become slow or may cease entirely.