Hey Top, looks like it's broken through the sealing "bead" around the top - and that's probably where your leak is coming from. Those things are pressed in, just like the block "freeze" plugs and can be replaced. They're getting "pricey" these days though, being as far as I know a Pontiac only item. I think I paid $50 + for one. Your idea about testing for the leak there sounds like a good one. If you find out that's where it's leaking from, then get yourself a replacement on-hand first, then you can drain the coolant (petcock on bottom of radiator) and use a hammer and cold-chisel to coax the old one out. Put a dab of sealant on the new one and finesse it in using a large socket on top of it and a mallet.