I never pay to much attention to rads unless they leak but on my Vega I've noticed it holds pressure for a long time where my Camaro doesn't. It's not just a little either,if I take the cap off it goes everywhere even if it hasn't run in a week.
Because my rad is lower then my intake I put a housing on the intake so that I can get all the air out when I fill it but I have the overflow coming on the rad cap.The cap on the intake is a 17bls. and the rad is a 16lbs. cap.
The car hasn't gone past 160 degrees so it can't be due to that.I tested the caps and they work fine and I don't see anything wrong with the 2 cap thing.I also ran the car up to 160 with the tester on the intake and it only went to 7 or 8lbs.
Anyone got any other ideas?

Cragger, that sounds normal. I'd say you are getting the Vega full with water. On the Camaro it is impossible to get it full without bringing it up to temp with the thermostat open and run the revs to 2000 or so while filling and then reinstall the cap at the same time.

t-stat making it hold pressure??

That does make sense Rick,it's does get filled completely for sure.I hope that's all it is.
1981coupe,I use a restrictor so no stat.