Another trick similar to one that Jerry mentioned earlier about using a dull drill bit is to cut about a six to eight inch piece off a wire coat hanger, then heat up the end with a torch and pound the end flat about 3/8" so that the end looks like a paddle. Put it in an electric drill with the paddle end facing out, then run it in and out of the drain hole. It works like a plumber's snake. The coat hanger wire is thick enough to hold the paddle shape on the end, but flexible enough to go up the water jacket.
On tough ones, I have found the combination of the dull drill bit, compressed air and the coat hanger snake to work.
If you are getting air to pass, you are getting close. The snake might help to loosen the last bit of crud that can't be reached by the dull drill bit.
Matt
