Hi Wenal,
It may be a leakage issue. A steering valve assembly has six big O-rings that are utilised to seal the outside. It's possible that one or more of these huge O-rings are leaking. The power steering housing can be replaced to correct this. After removing the steering wheel, you'll screw on the steering shaft in a clockwise direction. At this point, make sure both valve bodies are out of the housing and in good condition. This replacement procedure should resolve the issue. However, if it does not, the condition may be more serious. This could indicate that one or more of the construction's valves have failed.