Hi all, I have a grey market 2012 yanmar vio55-5b. It was running awesome until I lent it to a contractor for work. They ended up backing out my bushing and bending my main ram and boom cylinder. The machine shop repaired it and a mechanic put it back together without flushing the hydraulics as they said there was no need.... I bring it home and all of a sudden my left track under solo operation locks up on me. I lift the track off the ground and it runs fine in forward and reverse - I inspected for rocks or anything blocking my tracks. Nothing! I put the track back down and try it under load and it locks. If I use both my left and right tracks under load there is no issue in forward or reverse - just when my left track is under solo operation. I dopped all my hydraulic oil, clean the tank and replace my return and vacuum filter - I notice some crud and what looks like warn hydraulic hose pieces in the bottom of the tank. There was minor fine metal on the magnetic plug - but nothing too major. I put new oil in and all of a sudden I loose power in not only my left track but now my arm and boom swing function. My boom up/down is fine. my blade is fine, and my right track is fine. I pull the relief valves of P1 and P2, clean them and reverse them to see if the function changes -- Nothing ! its exactly the same. I put a pressure gauge to P2, and dead-end my boom - reading is 3600psi. I deadened my arm and bucket and it slowly builds to over 2000psi then immediately drops to 350psi. I changed my gear oil in the left track and seen nothing crazy - little crud. from my manual, it seems that all my functions that are weak come from the P2 pump. Any help diagnosing this would be awesome.
My guess is you have some bypass in the swivel joint and you may need new final drives. I have a similar machine 2010 vio55. the left final drive had low power if it was run solo like you said. I've replaced both final drives. It's much better.