Your cap will be in this position. Now push down very hard and continue to turn it. Bob DeKruyff suggests \"Now, if I try to rotate it back and forth a bit while pushing down, it will usually come right off. (usually). David Greenberg says \"I used some JB Weld and added an aluminum angle scrap which gave me a better grip\" Jim Kanomata adds "Frank Jenkins has caps with a tab soldered on it. gmcfrank@yahoo.com ". Ken Burton adds "Go to Autozone and as for a 9 pound cap replacement for an AC RC-32. Their cap has a hexagon top and removes easily. Cost is around $7.00. " Gene Dotson adds "Try a Stant radiator cap with the red release lever. You can get it in 7 lb. and 13 lb pressure. When you remove or install it lift the lever and rotate it with slight pressure." However I believe the GMC radiator requires a 9lb cap. Wayne Lawerence also has a photo posted here http://www.gmcmhphotos.com:80/photos/showphoto.php?photo=12570 .