Ok we need to establish some things.
What sight glass are you looking at?
Are you using oil or hydraulic fluid?
Where are you filling it?
You have a very uniquely designed tractor, it was a model at the transitional stages of the newer tractors and they had not quite gotten up to speed on things.
As such the Hydraulic pump is a 2 part pump and both parts are completely separate, the smaller rear pump is for the steering, and only the steering, and here is where it varies from most all other models, it uses it's own tank separate from the rest or the tractor, so the steering can work without anything else working.
Have you changed the canister filter for the main pump?
Is the transmission and rear end filled with the proper level and type of hydraulic fluid?
The reason that it's sounds like it's sucking air is that it probably is.
Do you have the WSM? If not drop me a note and I'll send you a link for a free download manual.
First pic is of Steering section and it's pump.
Second is of main hydraulic pump, #11 is the filter.