Thanks all for the support. Maybe the following will help someone else. I was able to cobble together a decent test system. I found an expandable plug at Ace for the top opening of the radiator. I found a bourdon tube dial type tire pressure gauge at Ace that read clearly in the range of 0 to 10 (and beyond) AND would allow me to unscrew the external piece that fit on to the tire Schrader valve. A few brass bits adapted it to a 1/4" barbed nipple. Amazon had a Schrader valve that terminated in a threaded fitting. I did have to turn down the tip of the valve slightly to get my air chuck onto it - my compressor does adjust to very low pressure, no damage to any cores. Some fuel line allowed me to couple this into the radiator overflow port. The results were good.

Bob

BobS46