The coupling is just getting rounded

thats what I thought too, yep stuffed it up worse....but if you can get a big enough one in there, you'd get more torque onto it compared to a little open end 10mm spanner. However if it has jsut as slippery of a grip, the greater torque will simply mean a greater rooting of the hex!!!
I wouldn't recommend doing that - sorry adam_rxturbo..... if you have buggered the shape of the hex then this means the steel has already yielded plastically and will not behave with the same strength properties as before. You'll proabably be able to do it up again - because tightening bolts generally requires less torque, but when you come to undo them again they'll deform on you again. Just get some second hand lines from a wrecker.And in the past i've found that once you get the lines off, you can pretty easily beat/squash/whatever the hex back into place. For typical bolts i just squash the hell out of the flats in a vice makes the hex like new again! But i wouldnt go squashing the hell out of a brake line bolt hex - or else it would collapse. But im pretty sure you can restore the hex atleast enough to be usable again