This problem will usually show up once the units condition has been let go. (many small leaks, or some not so small)
Air can enter the steering/transmission hydraulics when engine off, and then an air lock can result.

Bleeding the air out is only a temporary fix...until the next time the unit is parked for a longer period of time.

FIX ANY EXTERNAL LEAKS 1ST, Then you may have to reseal or replace the pump or shaft seal.

Note: When running the unit in this condition...the pump is running dry!!

By the way the recommended oil is J20A or J20C Universal tractor transmission (was an update in the early 80's)