I have a 1990 JD 70d that rubs great is smooth and strong. Bought it for the right price knowing about final drives being unavailable. In any case since day 1 it will track forward and reverse straight with no load. Tracks forward straight with a wet round bale in the thumb. In reverse seems the left hand track doesnt have as much grunt from time to time as the left. Plus when doing a sharp turn left hand track will lock but rarely move as fast as the right. Fluids good no metal in the filter. Would the swivel joint be a good place to start? And am I right in assuming I should check the pressure to each drive to ensure the same? Undercarriage has recent rollers and has never spat a track or skipped a tooth on me I just noticed this a lot more as I was wadding up balls of brush against the blade and tracking backwards to my burn pile. Saw someone who actually knows what they are doing move brush this way and figured I would copy him. If it is the center joint I assume it comes out the bottom with a shower of oil per the manual I have or is there a better way? This thing looks bad and I got it for a song as the old owner for some reason didnt address several problems in the fuel system and figured the machine was whupped. After fixing the fuel system and cleaning the rocks,dirt, water an cigarette butts out of the fuel lines and repairing the swing drive it does everything I need with no compromise including pulling acres of Scotch Broom. Roughly 5400 hours on the machine.
They can be a mess to deal with, the small machines have the same amount of hoses in a much tighter area. We have a 60 with a cab, and the only access is from the bottom unless the cab is removed. To help with the drive issue though, there are some fairly easy tests to determine if the problem is the motor or the swivel or the control valve reliefs, or even the pump. Most motors will have a case drain line which routes the leakage oil back to the filter and tank, you can take these loose and see if the oil flow is the same. The swivel can also have a bad seal. The line relief valves at the control valve could be damaged and allowing leakage. The swivel will probably be the culprit, but I would make sure before going through all the work of getting it out of the machine, they are installed first with the rest of the machine built around them, lol.