1- pressure regulating valve spring loose, such as pressure did not change after the valve may be used for a long time due to wear and tear caused by leakage can not afford pressure
2- hydraulic pump wear leakage for a long time
3- hot oil temperature is too high, the hydraulic oil is too thin can not afford high pressure
4- oil cylinder piston head seal wear or aging caused by oil and can not afford pressure
5- solenoid valve used for too long, spool wear, leakage caused by the pressure is not large
6- pressure check valve (if you have oil on the road) leakage
7- oil pressure pipeline, oil cylinder welding spot, piston rod seal
8- oil level, oil filter and air suction.