You would think a normal cap should work.
On my car, the coolant heats up, then goes to the overflow bottle,
from the cap hose.
Then, when car is cold, I open radiator cap, and level in radiator
is about 3 inches below cap. So, how did that air get into radiator,
if not through the radiator cap, which is plumped to the overflow bottle?
If air did not get into the radiator while it cools down, then
it would create a negative pressure, and possibly do damage to
gaskets, etc.