I do not know what in the world happened to my images, but I put them back!
This was my first test:
I sealed a funnel in one hole, put a rubber stopper in the other, and neither heater core leaked. I left them for hours.
This was my first test:
I sanded down a Schrader valve, put it in one hole, put a rubber stopper in the other, held them in place with mending braces, and used a bike pump to pressurize the heater core to 16 PSI, what the cooling system is rated.
Both leaked out within a few minutes.
I may have lost my images inexplicably, but the explanation was still there!