My boom is going slowly down when crane is parked. Is there any way to find out if it is the valve or the cylinders leaking?
It won't be the control valve. The oil is either bypassing the piston seal, or the holding valve isn't seating correctly. In my experience, its the piston seals most of the time. I usually replace the holding valve when resealing the cylinder. You can test to find out whether its the cylinder or the holding valve, it give the procedure in the manual. But you need to know what you are doing.