OK.... here's the deal at our end.
We are pointing you in the right directions but like any proper diagnostic procedure, there are many steps that must be taken to isolate the TRUE problem instead of the guessed-at suspected problem. Now, I could be just like some garage mechanics at this point and tell you to install a brand new pump and drive motor. I'm pretty sure that would clear up the problem immediately.......but at what cost? The proper way is to isolate items and check the performance. Perhaps the pump is OK. Perhaps the problem lies with the relief valve. Perhaps the problem is solely the drive motor. On this site, we don't play silly, expensive guessing games at your expense. Instead, we try to guide you through the diagnostic process..... which is far less expensive.
Personally, I don't go along with putting gear oil in the system because it really does not cure the problem. It's similar to putting a bandaid on an infected wound. Yes, the wound is now out of site ...................until your finger turns black and falls off.
As for rebuilding these pumps and motors, you are welcome to take a shot at it. In the opinion of many, many people....it just isn't worth it. Working on the geroler style of drive motors calls for skills and machinery that are beyond most individuals that are not already involved in hydraulic equipment repair. Of course, you have nothing to lose by trying. Perhaps there is nothing all that wrong with the motor. Perhaps the pump is OK. Until you come to the realization that you need to follow our advice and conduct the tests, you are flying blind.
As a mechanic, you should be well-versed in the merits of taking a compression test (wet and dry) and conducting a leak-down test in order to assess the condition of the valves and rings in an engine. Hydraulics is no different. You came here for help because you don't know your way around these tractors. We can't push you into accepting our advice. That's in your hands. All I can tell you is that the "experience pool" on this forum is very deep.