GE Fr. 9E 9171 main hydraulic oil pressure get change suddenly without change in parameter which can effect pump performance, line traced no leakage found. I use VR21 for pressure control, but no change. Finally, I twisted pump pressure compensator to increase the discharge pr from 93 kg/cm2 to 100 kg/cm2. After 24 hrs it again dropped to 92 kg/cm2, again without any reason. Everything is fine with auxiliary hydraulic pump (104 kg/cm2).