If you are settling still you have to check the outer cylinder drip tubes to see if fluid is bypassing here. This is the only path left for oil to return to tank unless externally leaking
You should be able to pull this line off at the tank to verify. Also you can pull the hyd return line off with unit off to check if leaking back through the control valve. Mast up in the air. The valve spool is locked so no fluid should be returning back here. Or use clear hose In place of return line. Rarely ever have these gone bad unless contamination has entered the system. Seen 1 in 24 years.
Last check is the solenoid valves on the end of the spools. Lift spool each one is separate. Overused fluid breaks down and causes ash build up and plugs up the ports.