Case skidsteer drive problem

I have a Case 85XT skidsteer that is weak in one direction on one side. It will go but it is sluggish. With lever pushed fully it is about half the speed of the other directions. At first I thought it was a bad relief valve so I swapped them all around and it made no difference. Then I thought it might be in the hydraulic motor on that side so I rebuilt it.....still no different. This machines hydrostatic pump is controlled with hydraulic servo pistons rather than a direct linkage to the swash plates. After some pressure testing, I found that the servo for that direction was not building up to full charge pressure (320psi) like the others were so I assumed the servo piston rings must be letting fluid get by so I installed new piston rings to that servo piston, still no difference in operation. Yesterday I removed the control housing for the servo and inspected the check balls/springs inside and everything looked fine. I even pulled the control spool which controls the metered amount of fluid to the servo and it looked fine. Now I"m at a loss as to what to check next. Can you think of anything else I need to check. BTW, it"s a Sundstrand hydostatic pump.

Thanks in advance for any suggestions you may have.

Thanks for responding, I need all the suggestions I can get.