Now that my problem is solved, I have to wonder.... This is probably a bad question for three reasons: (1) Blasphemy and (2) Initiation of endless debate, degenerating into name-calling, high blood pressure, and gnashing of teeth (3) It's probably already been answered. Given that, I have to go for it. For this class of small leaks in pressurized systems, what are the merits and/or risks of boiler 'stop leak' treatment? I used many similar products on old cars in my younger days, and they actually worked surprisingly well. Similar temperatures and pressures. Could I have solved this years ago for a couple bucks and a couple minutes? Would I have voided my warranty, ruined my system, frozen my circulators, and instigated a plague of locusts? So - what's the collective wisdom of the forum? For my part, any single circulator in my system is worth more than the cars in question (really - it was a long time before I paid more than $50 for a car), so I've shied away.