the radiator is a non-sealed system in regards to your radiator cap acting as a pressure relief valve and the tank is a overflow tank. As your vehicle gets hotter the fluid expands and has to go somewhere. Once the pressure builds up to a certain level it pushes the spring upwards in your cap and allows coolant to push out into your overflow tank. As it cools it will suck some back in. As long as the overflow tank wasn't dry the system hasn't sucked any air into the radiator. The green coolant was probably the cheapy glycol based coolant which is NOT the proper coolant. It will cause corrosion of aluminum parts. It should have either been pink or blue. HOAT based coolants only, hybrid organic acid technology.