Many of the failures in a hydraulic system show similar symptoms: a gradual or sudden loss of high pressure, resulting in loss of power or speed in the cylinders. In fact, the cylinders may stall under light loads or may not move at all. Often the loss of power is accompanied by an increase in pump noise, especially as the pump tries to build up pressure.
Any major component (pump, relief valve, directional valve, or cylinder) could be at fault. In a sophisticated system other components could also be at fault, but this would require the services of an experienced technician.
By following an organized step-by-step testing procedure in the order given here, the problem can be traced to a general area, then if necessary, each component in that area can be tested or replaced.