Hey Chip, I had a radiator that would seep a little only when it got to operating temperature. We changed radiator cap from 16lb to 14lb and was enough to stop it. Lasted about a year that way but eventually had to repair radiator.

Maybe that combined with expansion plug will get ya a little more life out of it.

PSO headed 632 from MM. Cracked cylinder, loose valve seats, low oil pressure, low cylinder pressure.
..... Now its a door stop....