Radiator boiling over continually

I have never tested to see if an upside-down stat will actually open.I mean the set -up to test it would not be the easiest. If it has the bleed-holes drilled in it, I suppose it could.Eventually...far too late eventually.
But as long as the fluid on the pellet side stays below the approximate rating, then no, the stat will stay closed. I imagine when the coolant begins to boil, the pellet on the cold side will be sitting in a mostly-air pocket, to which the pellet will not respond

I'd like to add that if the cooling system in our vintage LAs is up to snuff, it will run all day with the rad cap off. Knowing that, I run a 7psi cap, up here in southern Manitoba, even with the coolant running 205F or a little higher.. The hoses last forever. That cap has been on there since 99. I only know that it still works by the fact that it doesn't lose coolant. Well, not much anyway.