[DanEC]

Originally Posted by vatdevil

The coolant tank exists to provide a filling point and a means for air to escape out the system. Otherwise, air would trap in the radiator, since its not the high point.

The overflow tank is just that, a tank to catch overflow coolant so it doesn't spill out onto road surface. The valve is used to drain the overflow tank so it doesn't push out the weep hole. If coolant pushes out the weep hole you are probably overfilling the system. Only need to fill the coolant tank halfway.

Unless the overflow tank is actually a recovery tank. Then you could put a recovery system type radiator cap on the expansion tank and fill it full.