Try "water glass" (Sodium Silicate). It's available in quart bottles on Ebay at pretty reasonable prices. It's a very effective sealer on low-pressure cooling systems. I managed to seal a seep leak from the head gasket on the 503 GMC engine that I drove from Tennessee to the reunion in KC last summer, and Ed Pruss drove it on to Denver without any problems.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!