I have had this issue over many years. With any bolt touching exhaust components you are liable to have a break. I have used PB Blaster for many many years and I gotta say it rocks. I also have always used the same technique that Hemibigblack uses, loosen, tighten, loosen always seems to work better for me. Also, who in their right mind uses an air wrench when working on any bolt on a vehicle that is rusted or more than a few years old without using a breaker bar first if it is in a critical location? Umm, lemme guess, some knucklehead that hurries and does not take care because it is not his he's working on, and knows if it breaks he can charge you more because he screwed up as long as he tells you first "hey man I am a screw up and am gonna break your bolts off so it is probably gonna cost you more." In most cases a little caution and prevention can avoid these issues. PB Blaster over a few days first then off to the shop.