arno, the radiator overflo hose has to pick up the excess fliud from the bottom of the tank to work. in other words, you can have it enter from the top or side, but internally, it has to run to the bottom[or close] to be able to siphon back to the radiator when it cools. also, as has been said, the cap needs to be vented to atmosphere. that is a cool tank you got !