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Hoses and caps arrived today. After I install these, the entire cooling system will be brand new. However on a side note... I decided to pop the hood and compare the new degas cap to the old one. The old cap clicks tight with hardly any pressure or force. Maybe a single ft/lb? Whereas the new one needs some real force before it will click. Maybe 10 ft/lbs. Whether those numbers are accurate or not, point is the new cap tightens down very considerably more prior to clicking than the old cap. That combined with the fact that the rubber seal on the old cap is indented from the degas tank, I have a feeling that my problem has been in a cap that couldn't hold pressure.
I'm still going to install the hoses and engine fill cap, but I think I've found the culprit.