Yeah, you've got to get that rear block drain going. On my GMC I had a similar problem. I pulled the entire petcock off and was still not getting but a seeping of water. I just used a length of coat hanger and continued feeding water into the radiator while I probed with it. After a while a bit a big glob of nasty looking stuff came out, followed by a lot of rusty looking water. I continued reaming and flushing until I got a clear water stream. All this was on an engine that hadn't been run in roughly ten years. After that I put a block flush in it, drove it around the block to get it up to temp then let it cool. When I dumped the radiator and block I again got more debris out of the rear block drain. It took a few times to get it really clean. Truck runs cool now.
David Bush
