Dont know if this is relevant or not, but I had some brake calipers with the same sort of situation.
The line that conects the two sides of the calipers was damaged, so when I went to remove the brass insert I stripped it totally and I totally rounded it off, and tried everything to get it off and failed.
I took it to the maintenance techs at work, they ran it under hot water for a few minutes and got it really hot, then just used mole grips and it came straight out with the slightest of efforts.
maybe if you can get some heat into it, who knows....