I've been noticing that some FE setups use a surge tank spliced into the upper radiator hose. I may be wrong on this, but I guess some were factory too. Most of them seem like high dollar pieces for the Cobra crowd, but I was curious as to what exactly the thing is for. Looks like a straightforward expansion tank, but if it has a bunch of air in it, wouldn't that keep the cooling system from pressurizing? It also has a fill on top, I guess you don't need one on the rad then? I'm just not grasping something about what these little gems are for. Anybody want to clue me in?