I recently bought a John Deere 750J with 5000hrs. When I checked the inner final drives both were over full, under pressure and the oil had the smell of hydraulic fluid. From researching this is an indication that the hydraulic motor seals are leaking, but also that a case drain from the hydraulic motor could be plugged and causing the case to over pressure and push fluid through the seals. I haven't been able to find anything about a case drain filter on this dozer. If it has filters, my plan is to clean or replace and run the machine and monitor the oil levels before tearing into it. Is that the best approach? Does it have case drain filters? Where are they? Are there case drain filters for the hydraulic pumps that I should check also? Any input is much appreciated!
The inner and outer final drives are filled with Hygard hydraulic oil. If the inner is overfull then there are only 2 sources for this. The park brake piston seals or the hydrostatic motor. To answer your question about return screens, no there are no return screens.
Is there a way to test which one of the two is leaking, or is it best to just replace both?
It is most likely the park brake seals. It is possible that the inner final drive housing is cracked. I PM'd you.