The chassis service manual has detailed instructions for dis-assembly and reassembly of the hydrostat. If you have one on the bench, and you follow the manual instructions EXACTLY you can take it apart, inspect/clean, replace gaskets and seals, and put it back together. It is not a task for the faint of heart or mechanically dis-inclined. But it's not rocket surgery either.
I've done this to 4 or 5 units. Had one that was destroyed internally so I took it apart out of curiosity. (had nothing to lose, I figured). After that, I started going through them as part of my restoration process. A couple of the others had one or a few pistons that were plugged or sticking. Was able to return them to service successfully.
Keep your work area immaculate, keep track of the parts and where they go, and do not mix them up. Some look nearly identical, but put back in the wrong place or turned around, and it won't work. Be careful not to pinch the charge pump "O" ring on installation.
The service manual is actually very good on this topic.
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