John Deere JD300 Shuttle Sift boggs motor in reverse

[muely123]

I'm looking to make an offer on an early '70's John Deere JD300 industrial tractor (similar to Ag version 1020) that appears to be in pretty good shape, with one big exception: Reverse don't work! When I move the Shuttle Shift to the reverse position and then let out the clutch, the tractor moves forward a few feet, then starts to back up, then boggs the engine way down to a stall, acting like something is in a bad bind. The owner says the problem is somewhat intermittent; sometimes he'll get a good hour out of reverse before it acts up, and sometimes it'll happen right away. I think he's had a little luck fiddling with hydraulic filters / fluid... Any ideas? I'm not afraid to split the tractor if needs be, but it'd be my first time. Also looking for docs / manuals on the tractor.