I recently looked at purchasing a D7e dozer. The guy that owns it has had it for 10 12 years. 2 years ago he parked it in the winter, came back in the spring and started using it after one long push it wouldn't back up. He cleaned all the filters, and found crap on the bottom screen. He also Replaced the transmission charge pump, After doing all that . The dozer will move forward and backward now. But you have to hold on to the shifter in either forward or reverse to keep it moving. And it doesn't seem to want to steer that well. Does anybody have any ideas? Things to check
The main thing to check is the availability of parts for a machine that old! Some of them are worth more as scrap metal!
There are pressure port/taps to check all the hydraulic system functions. Low transmission oil pressure will automatically kick the transmission out of gear as a safety function. If your serious start by getting the parts and service manuals for the serial number and start checking things out.