[Rick Parker]

Caps that are intended for recovery type systems will have a rubber seal around the perimeter of the cap that seals on the top of the neck as well as on the smaller hole (approx 1.25" dia) into the radiator. As the coolant expands with heat and creates pressure it lifts/forces open the "plunger/seal" on the smaller hole and coolant exits the radiator and or surge tank. It will then flow throught the overflow hose and either onto the ground (non return) or into a sealed tank (return type) that will itself often have a radiator cap on it (rated higher than the cap on the surge tank or radiator). On the sealed system when the coolant in the engine begins to cool it "Shrinks", taking up less space in the radiator and with the system sealed it pulls coolant out of the sealed tank and past the metal button you speak of.