Very tight flare nut

[edwardmk]

New rear Mtec discs and fully refurbished rear calipers are back on the 190 at last. Final step is to replace the flexible brake lines. So I pulled out my new flare nut wrench kit and found the 10mm was too small and 11mm was a little too large. I put pressure on the 11mm wrench but felt it was about to round off the flare nut. It's been soaked in 'Ferrosol' for at least a week and cleaned thoroughly. Is it possibly an imperial size nut? I'm thinking that if I tear something I should be prepared for replacing a section of the rigid brake line. All advice very much appreciated.