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Bob1951: Comments?? Where is Eric?? Always enjoy his comments.
One thing to remember is that the cylinder when running isn't always under pressure, there's an intake stroke that operates under vacuum remember.
BUT....
The 1/5 a second that it's under vacuum isn't very much time for it to allow pressurized coolant into the cylinder, which is why it doesn't suck all the water out of the radiator right away. The likelihood of cylinder pressure going into the water jackets is a lot higher than coolant pressure forcing coolant into the cylinder. Both happen in a blown head gasket situation, but cylinder pressure into the coolant happens a lot more simply because more pressure is involved. 100+ psi Vs. 14 or so.
Think of it this way, try to inflate a tire that's at 20 psi to 35 psi with your mouth, pretty much the same thing.
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