Not Msammy here, but as long as your mechanic can get back to where the lines are solid on both sides of the rusted line, it isn't a big deal to replace a rusted section with a new piece of brake line. All he needs is the flaring kit and fittings.

He said it depends on if both are rusted or just one. Would it be better to use lines from the dealer or aftermarket preassure fitted ones for the fix?

BMW doesn't make brake lines, they buy them from a supplier and form them into the precise shape. The only advantage to buying them from the dealer is that they are pre-formed, so if it is a complicated shape it may be cheaper to buy it from the dealer than pay your mechanic to make a straight line fit, using a tube bender so that it isn't kinked. It should be your mechanic's call, he can see what the most appropriate fix is.

Ok thanks, i am dropping it off tonight, he will work on it tomorrow.

Will update then.

I had similar problem when my DSC Pressure Sensor was loose. It sits on the Big aluminim Block.

Well both lines and the connectors are rusted, i will post pics tonight but my indy mech said that it would be more work to replace the lines since the whole rear suspension would need to be dropped ($1500 job) to get the lines out/in. He is going to use pressure fitted and bend them as needed. I understand why now you can't just replace the oem lines.

Lines patched. $60 in metal tubing and $280 for labour. Not bad at all!

As promised here are the pics, stupid cdn winters i have to thank for these:

http://i51.tinypic.com/1z1d0kn.jpg
http://i54.tinypic.com/elba03.jpg
http://i55.tinypic.com/2lsadqg.jpg

More to do with the salt that your local government uses on the roads.

Shouldn't there be plastic coverings?