i have always had excessive pressures in my cooling system since i have gotten my car running. the upper and lower hoses used to be rock hard after warm up, and it always left me uneasy. everything in my cooling system is new, the most recent additions are koyo rad, new upper and lower hoses, 18" e-fan, and spal PWM controller. after installing everything i checked the hoses after a good proper bleeding and the hoses felt like they were about to burst. it never overheated, which is why i was never too worried.
well today my friend came over to my garage and we were going through all the possibilities. i thought it was the rad cap, the overflow hose location, all possible ideas. it turned out that the overflow tank cap was clogged! it ended up being clogged right where the cap tube makes a 90 degree bend into the bottle. there was gunk blocking so no amount of blowing would clear it. i used some drill bits and just dug the crap out until i could get air to pass through.
i just wanted to share my story, hopefully this experience will help those with the same symptoms. it just goes to show, don't overlook anything. that little blockage caused my cooling system to become overpressurized and when the cap reached it's pressure, it wouldn't expand.