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I've read all the posts here. After reading the OP post I thought it could be a head gasket or crack. Didn't think about water pumps cavitating but I do know that all kinds of pumps can cavitate...probably not the case here since they haven't been cavitating for 80 years. A radiator needs to flow cleanly...if it's blocked to a point, the pumps will be sucking and the radiator won't be able to deliver what they want. The coolant that does get by builds up on the pressure side (upper) and it would get blown out of the cap or tube. There's tests that a shop can do to see if there's any exhaust "residue" in the coolant. That will answer most of your questions. I think that test is not very expensive either. Rule out one thing at a time but I think a restricted radiator can cause the problems you're describing.