[Stocker]

If they do, that's a new one on me, I sure wouldn't think so. That said, I can't think what might be causing your situation.

A guess or two -- With your recent cooling system work, maybe there's air trapped in the system? Have you tried drilling a small (1/8") hole in the t-stat? Some guys say it gets trapped air out of the system, others say no. Either way, it doesn't cause any problems and *might* work as claimed.

To your last question, an overflow reservoir is a great idea and I would not be without one. Besides capturing and reclaiming any leakage, it lets you run a full radiator, which can help keep things just a bit cooler. Make sure you get a radiator cap for a sealed (closed) system or it can't pull coolant out of the reservoir as it cools.