If you have absolutely verified that the charge pump is not the issue, then the unit has to come out and come apart. I rebuilt hydraulics for a living for twenty eight years and have seen most every failure possible, I'm sure. Including trouble shooting after the rebuild. The U15 Sundstrand is a very simple hydrostat, but even simple won't work if something "simple" is wrong.
The most common problem with your symptoms is the lens plate (brass) is not seated flat on the head. Or damage to the lens plate rotating surface. Did you "polish" or sand the lens plates?
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