What should I do if the piston rod of the excavator boom cylinder, arm cylinder and bucket cylinder only extends but does not retract

After the excavator is used for a period of time, the piston rod of boom cylinder, stick cylinder and bucket cylinder may only extend but not retract.
The pressure of the valve block and oil pipe may increase abnormally due to the blockage of the main return oil circuit. The load of diesel engine is increased. In case of failure, first find out the cause of failure step by step. Check whether the valve core is in the normal position. If it is in the normal position, the main reason for the fault is that the shuttle valve is installed reversely. By analyzing its working principle, it is known that the combined valve of the front feed circuit will drain their respective working pressure oil according to their different working conditions. Push the speed limit valve through the shuttle valve. Open the main return oil circuit to keep the whole hydraulic system in normal working condition. That is, all valve groups and all actuating elements can feed and return oil normally. If the oil inlet of the rear group valve on the shuttle valve is connected to the oil outlet of the shuttle valve by mistake, and the oil outlet is connected to the oil inlet of the rear group valve, the speed limit valve on the main return oil circuit cannot get the working pressure oil drained by the shuttle valve to control the spring force of the speed limit valve. The speed limit valve core cannot be pushed to the oil return position. At this time, the speed limit valve core is in the position of closing the main return oil circuit under the action of spring pressure. Because the pressure oil output from the hydraulic pump has no circuit, the pressure of each pipeline of the valve group rises abnormally, causing the oil pipe to break. If the reversing valve is pulled in this case, the volume of the rodless chamber of the hydraulic cylinder is large and the pressure is high. It is also possible to push the piston rod reluctantly but cannot retract. Therefore, reassembling the shuttle valve can solve the problem.