From: "Jim Grammer" <jgrammer@domain.elided>
Replace your radiator cap, I prefer 7PSI.
From: "Allan Ericson" <lowvolt@domain.elided>
I'm with Jim on this one. Every IH I've seen with that problem had a cap
that was 13, 15, or similar psi. Swap to the 7 psi cap, problem has always
gone away.
From: Steven Stegmann <steve.stegmann@domain.elided>
... slight dent in the sealing surface for the rad cap. ...
Check for leaks somewhere. If everything is tight,
there's enough vacuum to make it work. But any vacuum
leak will kill it.
+
The vacuum portion has nothing to do with the pressure rating of the cap.
If you need a 7 psi cap, something is wrong with your system.
The problem was the same way back before the rad was re-cored, when it ran
a 7 psi cap to avoid over-stressing the tired old seams.