We all know old cars, or those in harsh nature conditions often suffer of having seized nuts by corrosion in the brake lines to calipers and cylinders, and even in the master cylinder. As I live in an island where sea breeze actually crosses the country north to south! "everything" metallic that can rust "will" rust relatively fast. So my hobby car has this problem: nuts in the brake lines are seized. Some of them even rounded off by previous owners trying to remove them. Independently of the means to have the nut out, I wonder if applying light paint to the nut threat, or even a soft touch of light grease would protect it somehow from get corroded and seized again. Both parts, caliper or cylinders and nuts are "rusting metal". Actually I don't know it is a nut or a bolt.
Grease and oil are the best method to delay rust of steel. In the old days of chrome bumpers ,etc, I would add some gasoline to grease to make it liquid and wipe it on the chrome . It would penetrate imperfections in the chrome then the gasoline quickly evaporated and left the grease ."Conversion" coatings ( with phosphoric acid like "Navel jelly") will be some help. Use it first then grease/oil. Basically seacoast environments are the worst for steel corrosion so the long term answer is to move.
If you are replacing each brake line, try this. Install and tighten normally. Put a drop of penetrant on the exposed threads, it will seep in up to the seal point. Wipe the fitting dry. Then roll some plumbers putty (oil based) into a bandaid shape and wrap the fitting. Squeeze it by hand to seal the edges. Monitor it and add a cover of cloth based (hockey) tape to protect it if needed, or for looks.