raise the mast into the area that it seems to drift the most. useing some hose pliers or needle nosed vise grips pinch closed the main return to tank hose between the tank and the valve. give some time for the void to fill and see if the mast stops drifting. it will stop if the valve is the problem. it will not stop if a cyl is the problem. the alternative is a drip test which is to raise the mast remove the return hose and catch the oil. spec's are available for volume / time.