Just finished replacing my oil pump housing "o-ring", crank seal, and radiator swap to the brass model. Figured since the old radiator was leaking, I would un-bend all the tabs on the top reservoir to see how dirty it was. It actually was pretty clean, but I noticed that the opening for the little hose nipple was completely plugged with "gunk". I used a very small wire through the nipple and pushed out a .25 inch or so solid plug of sludge that looked like a piece of spaghetti noodle. The plug was not solid when pinched and fell apart, but would have been a complete blockage to coolant flow from the little hose. So my question is: What is the purpose of that little hose and coolant circut? FWIW: I completely drained the coolant system when I bought this rig as a PM last spring. Used distilled water as a rinse, and refilled (green by the way ). Coolant looked new when I drained it this weekend. There was no evidence of sludge that I could see in the rows. Maybe this is something to check when doing a cooling system PM? Pull the hose off and see if you can blow into the radiator via that nipple.