Dear Townshend,
Hope you're well and stay so! You wrote, "All the rudimentary bleeding methods have been employed...".
Does this include parking on a steep incline (e.g., a hill) and allowing engine to idle for, say, 10 minutes (nose towards hill's summit and then another 10 minutes with the tail towards hill's summit)?
I ask, because this method usually allows all air to exit cooling system, after the replacement of coolant, water pump, heater hoses, or rad hoses replacement.
Hope this helps.
Yours faithfully,
Spook
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