Good to hear from you, I believe I have talked with you before of this matter.
This tractor has the pump on the side of the engine and also operates the power steering. 4 speed gear box, no high-low.
The idea of it getting air into the system sounds like the problem, but have gone over this several times before and nothing. There are areas that are very hard to get to, loader, tractor frame, etc. but I will go over it again.
There are times that after having presure it will push air and oil out the vent which is between your legs at floor level.
I got my service manual and they spoke of Directional valve possibly the problem, not sure of that. When ever I start the tractor gear shift in neutral and shuttle lever in neutral.
I do have stupid question as no one seems to know, oil level on dipstick on full, when at temp, running, in neutral with pressure, or when cold not running?