[mrmustang]

How old are the upper and lower main radiator hoses?
Could the lower hose be old enough to that it is softening up and potentially collapsing under a load of hot coolant being sent through it?

or

If it is new (or old for that matter), does it have a stainless steel spring installed to prevent it from collapsing. If it's an older hose with a spring, could that spring now have disintegrated to the point that it is allowing the hose to collapse?

Just a thought from someone who has been there before.

Bill S.