On the right side where the hood ends at the dash panel. There is a rubber hose and it had to be removed to split the tractor. Check the clamps and cracks in the hose, You must be sucking air into the system, that hose is known to crack.
So far, I think you hit the nail on the head. The dealer reported that on that model that hose was known to crack and let air into the system. They're coming Friday to fix/replace the hose.