Agree, engine running or not should have nothing to do with loader leaking down.
Assuming we are talking about the loader lift and not the curl, and nothing is leaking externally, the only thing it can be is the valve. But spool valves always have some leakage by the spool. There should be an acceptable amount as per the MFG. Usually equates to somewhere in the ballpark of the loader drifting ~1-2ft per hour. Less than that and it would probably be in the "acceptable" range. More than that....valve is worn out.