I agree it makes no sense. I bought a new overflow cap and it still leaked so I bought a second new cap to be sure and this one was blue/green like the original one that I had on there. The newest one also leaks. The strange part is that I still have pressure in the system the next morning after not driving. The fluid isn't hot but there is pressure.

Before you mentioned burping the system. I took the cap off and squezed all the hoses to get the air off. Is that enough? I have leaked over 3 gallons of fluid out of the cap so by now I would think any air bubble would be gone.

Early tonight I ran the engine and I am pretty sure that I can hear the thermostat working, of course it may not open all the way. I may take it out this weekend and do a boil test. I still don't see how the pressure would remain after the engine cools if the thermostat was the problem.