I am not happy with the rate of leak down of the backhoe bucket, that is why I started this thread hoping to gather thoughts of others. I bought the unit new and had several issues with the backhoe. The selling dealer picked up the unit and held it hostage for six weeks, then said they replaced the cylinder in question. I have my doubts about that replacement, there were no signs of work. For some reason the cylinder held when I picked it up, so I brought it home with none of the other issues fixed. Once home, the leak down resumed.
I took it to another dealer (Messick's) where they rebuilt the cylinder and saw no obvious internal problems with it. They did say that oil was bypassing when tested but maybe they still had the spool involved. They weren't able to get parts to fix the other problems blaming a lost shipment. Once again I took it home after two weeks and the leak down resumed, but not as fast. I haven't timed the leak down yet, my iPhone sent the photos to iCloud purgatory along with the embedded times. My guess from the beginning is that there is debris or other issue with the spool involved.
I'll time the leak down under various conditions as I get a chance.
So your issue is the backhoe bucket rotating and not the boom dropping? This is interesting to me because I have noticed with my backhoe, the boom is more likely to drop faster than the bucket does, simply because of the amount of weight the boom cylinder feels, as compared to the bucket cylinder, when the backhoe is at rest.
Seems (if the general circuit is intact without leaks that would show up on the ground) the issue is likely either the control valve or internal cylinder leakage.
Since you had the cylinder rebuilt, you seem to be on track suspecting the control valve.
Just to satisfy my curiosity though, will you verify you are talking about the bucket rotating downward, rather than the boom falling downward over time. In my case, it was the boom, but I just took five seconds to put the boom lock in place and it was never an issue. BUT also in my case, the boom stayed in place for a pretty long time before it fell enough to be noticed. Probably at least an hour, but I never timed it.
Bucket rotation due to gravity pull on the bucket? Probably days, not hours or minutes...