Upper hose sucked shut.

[SicMonte]

I am having a major cooling problem. My upper hose is sucking shut after I turn the car off and it sits for a little time.

This started happened about two weeks ago. Since then I have put in a new 185 degree thermostat (turns out my old one was 160??), new upper hose with a spring inside it, new 15lb cap, and I even had my griffin radiator tested and repaired (they reinforced a core and added some extra epoxy around a small possible leak). After puting in all new coolant I drove it around and it was getting hotter than it usually does (im guessing this is b/c of the higher thermostat?). I brought it back to the house and parked it. The upper hose was very hot...and the lower hose was warm to the touch. Fan works fine. I walked away for about 30 minutes to let it cool and came back. Both hose's were fine (upper with a spring and lower is copper cool flex) so I took off the cap. WOW!!! That thing just didnt want to come off!! Tthere was so much vaccuum in the radiator that it actually sucked off the rubber ring on the cap while I was taking it off. That is just not normal. What could cause this??

Doug