I just opened up the HST on a 3600. It was full of really thin, black oil with much metal flake. I cleaned and flushed the unit and could not any significant damage. Seems the metal flake came from general wear. The dirty oil caused the push rod for the bypass (manual moving) to be stuck in the open position. I will be re-installing the hind quater in the next few days. Hope this solves the problem of slipping and not being able to move either forward or back.
Is there a HST parts list or diagram somewhere?
Sounds like that would cause a lot of problems. If you look down in the posts, I have a thread entitles "parts diagrams" where there are a couple links you can look at. I don't know how far they get into the Hydro, as I haven't looked at that before.
Good luck with it & let us know if that worked for you.
After cleaning and reassembly it still did not work, today (Monday 5/11) I brought it to a hydraulic repair place, the guys said that over time and dirty lube these parts wear down and allow oil to pass by that defeats the whole purpose of the unit. They said they will look inside and "lap" the surfaces and replace seals if it's worth it. Might be hard finding parts if needed. If all else fails I will part the tractor out on this board.