I am upgrading to rear disks and switching to the dual MC and cannot get the brake lines to break loose at the distribution block. I could cut them off and try to reflare them but there is very little room with the Hyd. clutch master cylinder right there in the way. Any ideas?
Common problem; brake fluid is very corrosive. That's why you should regularly flush the system even if everything is working fine.
Now, to your question; I use a product called PB Blaster. It can be found at Autozone and other parts stores. It is a can of lubricant/deruster. Spray it on and it will creep into the threads. For a tough job like this it helps to lightly tap the fitting with a hammer. This breaks the rust a little allowing the penetrant to work even better. After doing this (soak it), let it sit overnight. Spray it again the next day and try to loosen it with a FLARE NUT WRENCH. If it is already rounded, you will have to resort to Vise Grips.
I second everything mustangdave said. And you should always use a flare nut wrench on brake, vacuum, etc. with these type of nuts. If not, sooner or later you will strip them (although it takes awhile...I do learn from experience).