I replaced the three flex "rag" joints: the filter input and output, and the branch up to the main pump, which meant taking the main pump suction line off, but not the steering pump. I left that last hose on, as it had been changed at some point already (straight heater-hose rubber vs the rag hose, but solid). That was good, because the steering pump input lines are a ***** to get to. Changed the main pump inlet O-ring while it was open. The old one wasn't leaking but flat as could be, no flexibility left. No surprise, 25 year old O-ring, I'd guess.
Filled it back up (TSC UDT, by the way, always works for me) and voilà! - instant success with a cold system. Steering and motive restored. No chattering implements, no foamy oil. To note, these hoses did not appear leaky, but must have been at least partially permeable to air. Hope this helps another orange tractor driver!