Some "zero pressure" cooling systems have a canister on the top tank of the radiator with the overflow tube attached to it. It's about the size of a small cat food can. Others have the overflow tube incorporated into the filler neck, and the radiator cap does not have a spring loaded center piece that allows cooling system pressure to build up. Then there's the pressurized system, with a gasket surface below the overflow tube, and a spring loaded centerpiece on the cap. Every pound of pressure on a cooling system raises the boiling point of the coolant 3 degrees (like a pressure cooker)- - - - -so a 4# cap keeps the coolant liquid to 224 degrees F. A 7# cap gets the boiling point up to 233, and a 15# cap will raise the boiling point to 257 degrees. (all at sea level). The downside is that most stovebolt cooling systems won't handle over about a 7# cap- - - - -the heater core is the weak point.
It's also possible to mount an overflow tank up close to the top radiator tank level, put the overflow tube into it, and keep the end of the tube submerged under the liquid level in the overflow container. As the coolant expands and contracts, the liquid isn't lost- - - -it cycles in and out of the recovery tank.
Jerry
