Unvented Radiator Overflow

:confused: I noticed that about half the aftermarket radiator overflow "catch cans" you can buy are not vented. So, if they are not vented, then how can the excess steam/water get in there? Wouldn't the pressure in the catch can be the same as the rest of the system?

For example, I'm using this...

http://www.summitracing.com/parts/sum-g1478/overview/