Overflow Tanks Needs to be Air Tight

To get coolant back into the radiator from the overflow tank, the rubber line from the top of the radiator to the overflow has to be air tight. No cracks, etc. If it is not airtight, air is pulled back into the radiator, not coolant from the tank. Also, the radiator cap has to be working correctly. It basically has a two-way valve in the top of it. If the radiator overheats, the pressure goes up and at a certain pressure the valve opens and coolant flow thru the rad cap, thru the rubber tube, into the overflow. When the rad cools down, the creates a partial vacumn in the rad which opens the valve in the cap in the opposite direction, allowing coolant to be 'sucked' back into the radiator from the overflow tank.

So, you can see that the radiator cap, it's valve, must be working correctly and there can be no air leaks in the rad cap seal to the radiator and in the rubber hose to the overflow tank. And, in the overflow tank, there is a tube that goes almost to the bottom of the tank. It must be in place, and the hose connection to the top of this tube must not leak, so the coolant will be pulled back into the radiator.

Oh, and all the other rubber lines in the cooling system must be air tight. If you have a small leak in any of them, rad hoses, heater hoses, etc, then when a motor cools down, air will be pulled into the system at the leak, and it will not form the vacumn to pull coolant back from the overflow tank.