If the overflow tube attaches to the filler neck of the radiator just above the top tank, the system is designed to use a low pressure cap- - - -4-7 PSI, for instance. There will be a sealing surface below the overflow tube for the spring loaded cap gasket to seal against, and another (sometimes brass) seal above the overflow on the cap that keeps the coolant that gets past the cap from leaking out onto the tank instead of going down the tube. The actual zero pressure radiator has the overflow tube attached to a round canister on the back side of the top tank and a sealed cap without any springs.
Jerry
