Repro RC-15 Radiator Cap Problems

[Ark68SS]

I'm the guy Bill R is talking about. What happened to me is when I stopped to gas the car up, coolant puked out of the overflow. When I opened the cap to add coolant, I inadvertently turned it to the fully open position instead of the pressure release position, and that's how I was burned. Totally my fault.
HOWEVER, the reason the car was losing coolant is because the cap spring had rusted and lost tension, and would not hold full pressure. Pics of the cap and more detail can be seen here-
http://www.chevelles.com/forums/showthread.php?t=298116
I've since bought one repro RC-15 cap (from Tamraz) and tested another, and neither of them held 15 lbs. One was 9, the other 11.
I'm currently using a Motorad brand cap I bought from Ausley's at Chevelleabration this year, and so far it's holding the correct pressure, which, to me, is more important than correct markings. :-)
I hope my experience will prevent anyone else from being injured.
BillL