Brake Proportioning Valve question

Maybe remove the nut, cut it off, slip on a new nut, re-flare the tube, and see if that doesn't fix the leak. A new flare nut fitting costs about a buck at most parts stores, but a new proportioning valve is NOT cheap by any means.

Kind of reaching here, but you might even try a little Teflon tape on the nut you have on there right now to see if that helps things. Clean the stuff off with brake cleaner after re-assembly so you can see if there's any fresh leaks.