That uses a HydroGear 314-0510, have you replaced the oil and filter???
Also the manual covers how to purge the air from the system, see attached, You have to do this when changing the oil/filter...
You should have No trouble removing this transaxle... And to change the oil you need to remove it.
Then locate the pipe plug on top of the assembly, remove the plug and dump out the oil. Replace with about 80 oz. synthetic engine oil 15w 50 or 5w 40, Mobil1 or Shell Rotella T6....
Follow purge instructions.... :smile:KennyV