Using my own car, a 78 98 as a reference. Maybe I'm missing something but since the overflow reservoir connector is on the top of the tank, how does it go back into the radiator? The manual says the add coolant to the reservoir so surely there must be a way.

 

Under positive pressure during overheating coolant is expelled. As the temperature lowers a vacuum is created which draws coolant back into the radiator. Temperature & Pressure are directly proportional - as pressure increases, temperature increases & vice versa. As temperature decreases pressures decreases. This is the ideal principle of gases, yet under a closed system such as the coolant system, temperature is directly proportional to volume, while pressure is inversely proportional to volume. Thought of another way, as the temperature of the liquid (coolant) increases, molecular (kinetic) energy increases - much like a pot of boiling water (or, a pressure cooker on the stove). This increase in both temperature & (kinetic) energy results in an increase in vapor pressure. The volume would actually increase under these conditions. Albeit, when the temperature & pressure of the coolant increases in a fixed volume container, the overflow (coolant) reservoir allows the volume to expand into the reservoir. As the pressure & temperature return (creating a vacuum), the volume decreases & fluid (coolant) is drawn back into the radiator.

In the simplest of terms - as the temperature & pressure increase, the volume increases (coolant is pushed under positive pressure) into the reservoir. When temperature & pressure decrease, the volume decreases and coolant is drawn back into the radiator via vacuum.

 

Make sure you have the correct "coolant recovery" type radiator cap.. so the coolant from the reservoir will get sucked back into the radiator..

 

The cap on the overflow tank should have a hose that runs down to the bottom of that tank so it can suck coolant back into the radiator once it cools and contracts.