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.

Yes definitely the hyd. oil leak from some way. since all the cylinders have repaired, the only place oil leak is through control valve to the tank. please double confirm with the control valve. have the spool for lift in center position? Or the simple way by sway the control with other forklift!