I recently swapped in a new DSM style radiator that's a sealed type rather than one with a radiator cap. As a result, I had to also swap in a DSM style water neck with the provision for the radiator cap. Ever since I did the swap, the radiator cap seems to be working funny.
As the car heats up, the coolant expands and the cap lets coolant bypass into the overflow tank as it should. I can visually see the fluid level rise. However, the cap is not allowing coolant to return to the cooling system as the car cools. If I shut the car down and let it sit, the coolant level in the overflow reservoir stays the same, even if the car sits overnight. What's really crazy is that if I take the radiator cap off the next day, it literally pops and I can hear the vacuum that built up in the system overnight release. This should be the vacuum/negative pressure/whatever, that draws the coolant back into the system from the reservoir tank, but it's not doing its job.
The system obviously holds pressure and is leak free. Aside from the scenario above confirming that, I pressure checked the system up to 20 psi...Good to go. I've tried 3 radiator caps: a 13lb Stant cap, a Mishimoto 19lb cap, and also a Greddy 1.4 bar cap. Same deal with each cap. The T-stat is new, and the system functions fine aside from this issue.