It is very common for seals to not seal very well without a pressure differential.

I would check the seals on radiator cap very closely for small cracks/tears and debris, especially the pressure relief valve gasket. I have found several bad caps, that were causing me problems, one had debris on the seal that was hard to see, and one was cracked, neither would draw coolant out of the resevoir. The latest, was on my 300D, and I think it may have masked the early cold pressure symptoms.