Chuck , I retired from a Clark dealer after 32 years and been in forklift repair for over 44
Years I have never seen a trans tank on a c500
The fluid reservoir is in bottom of transmission
Your leak is possibly a screen seal or a bell housing. the petcock you are referring to is probably the old fuel tank the fuel tank and hyd tanks were built in to frame. sometimes
when they added or converted to LP they would put oil in old fuel tank to discourage rusting
I would clean off as much residue as possible and then check to see where the leak is

Much luck