With your model number, I was able to find the manuals for your machine.
For sure there is little info about the hydraulic system in the operators manual.
As I was looking thru the parts manual, I see where there is a single hyd oil reservoir and a filter.
It appears from the parts manual that the hyd oil filter is mounted horizontally, and if so this complicates getting rid of the air in the system that will be trapped in the filter until it fills completely.
The charge pumps on the hyd pumps draw in oil thru the filter and if the filter has trapped air in it, this air will go directly into the hyd pumps when the engine is started. The charge pumps are directly mounted under the cooling fans on the top of the hyd pumps. The same shaft that the fans are mounted onto also drive the charge pumps.
If the filter actually is mounted sideways and has flexible hoses connected to it, then possibly the filter mount may be unbolted to allow installation of a new filter that has been prefilled with oil and reinstall the filter and mount back into the normal position.
Bear in mind that when the filter is removed, all the oil in the hyd reservoir will drain out thru the filter mount.
Let us know what the situation is with the filter position and possibility of temporarily removing the filter mount.:smile: