Those original heater core will begin to expand out at about 7lbs. Any more than that and
it will damage the tubes which were very thin and designed for no more than about 4lb
operating pressure.
Got that from a radiator guy that I've known since high school, we were class of '60 and it
was a family business which just closed a few years ago. I also did some pressure testing
on my own and I can guarantee you that it you try going past 7lbs you will hurt the core.
http://www.pbase.com/dennygraham/image/113977758 Denny Graham
Sandwich, IL
