V8 conversion cooling basics 101, Expnasion tank to overflow tank to...? Options

[BRAVE_HELIOS]

Cooling experts. help!

I will premise my story by saying that I will be converting my mechanical water pump to an electric unit, but I wonder if my issue goes beyond swapping water pumps.

So I fill my overflow tank to just above the add line with coolant (expansion tank is not opened at this time) when the engine is cold. I drive the car and the temp creeps up to around 190-200 but not much more. Coolant level starts to rise in the overflow to where it flows out of the overflow tube and on to the ground. Eventually it stops pissing. The overflow tank is fairly full at this point. I leave the car alone until it completely cools off. I go back to the overflow tank and notice it is bone dry. I add more coolant to just above the add line in the overflow tank and drive and the whole thing repeats.

Is this some sort of tell-tale sign of something else going on?

Although it just does not seem right, what if I leave the overflow tank empty?

Perhaps a bad pressure cap on the expansion tank?