Just remember that the radiator cap is essentially another thermostat that is set to open at a certain PSI. If it doesn't open, the coolant won't have a place to expand when it's at operating temp.

Too see how it work's just put it on the bench and fill it with some water. Push the plunger down and the passages will be exposed and the water should flow out. This is how the coolant from the overflow tank flows into the rad.