That depends on why you are replacing one line. Typically there are four steel lines, one going to each wheel, but there are variations on the master cylinder design. A few have all four steel lines leaving the master cylinder. If a rear line rusts out and starts to leak, it makes sense the other rear line is just as rusty and should also be replaced. The front lines are better protected from salt and water so they cause less trouble. They most commonly rust near the end where they attach to the rubber flex hose right by the wheel. Often it's not practical to try to replace an entire line from the master cylinder to a front wheel. Instead, cut the line in an accessible, rust-free area where you can slide on a fitting, then make a double flare. From there, install a union and a short length of line to replace the rusted section. For best success making the double flare, do that in a section of line that is straight. If you try to straighten a section that had a bend in it, the flare will form off-center and may not seal.
Most cars have two steel lines that leave the master cylinder and go down to a combination valve. That is a brass block that sits on the frame rail or body right under the master cylinder. Those lines rarely rust out or cause trouble. The most common problem is the soft metal line nuts rust to the lines, then the lines twist off when you try to remove them to replace the master cylinder.
As a side note, it is extremely critical that brake fluid never gets contaminated with a petroleum product like engine oil, power steering fluid, transmission fluid, axle grease, or penetrating oil. Do not use penetrating oil to try to free up a line nut that is rusted to the line. Instead, break the nut loose, then use a propane torch to heat the nut and expand it. Dribbling water on the line may help to shock it and shrink it to break it free. Once you can start to spin the nut a little, spray some brake parts cleaner on it while working it back and forth to free it up.
A lot of people cause more trouble when they bleed the system. I have had the best luck with simple gravity-bleeding. Loosen the cap on the reservoir to prevent vacuum from building up, then open the bleeder screw and just wait for the fluid to show up. Once one wheel that needs bleeding has fluid coming out, close the bleeder screw, then wait for the next one to start flowing. When they are all done, "irritate" the brake pedal a little by hand, then open each bleeder screw once more to expel the few remaining air bubbles that washed into the calipers or wheel cylinders.
When people think they need to pedal-bleed with a helper, the brake pedal usually gets pushed all the way to the floor, and that can damage the master cylinder. Crud and corrosion build up in the lower halves of the two bores where the pistons do not normally travel. Pushing the brake pedal over half way runs the rubber lip seals over that crud and can rip them. Typically that causes a slowly-sinking pedal, and that often takes two or three days to show up. To avoid this, never push the pedal more than half way to the floor unless the master cylinder is less than about a year old.
Most older rear-wheel-drive cars and light trucks have just one steel line running to the rear axle, then it splits on the axle and a line runs to each rear wheel. If a nut is rusted to the line on the axle, don't bother trying to save it. Just let it twist off, then replace the entire line. If the line is rusty enough to leak, it is likely the bleeder screw is too rusty to get loose. If you have a rear caliper, unbolt it and hold it with the rubber flex hose connection up, then loosen that to bleed the air out. For rear wheel cylinders, the best solution is to just replace the cylinder, (they are not very expensive), but if that is not an option, remove one brake shoe, the wheel cylinder's dust boot, then the piston. That leaves the rubber lip seal still in there. Use a nail or piece of wire with the end smoothed to remove sharp points, and push it over the top of the seal to create an opening on top for the air to come out. This works best if you push the seal in on the bottom first to tilt it. That makes pushing the nail in easier and less likely to cut the seal.
Almost all front-wheel-drive vehicles use a split-diagonal hydraulic system where one front brake and the opposite rear brake are on the same circuit. That means there are two lines running to the rear brakes. Additionally, minivans and pickup trucks have a wide range of loading variations, and that requires modifying how much braking power goes to the rear wheels so they do not lock up under light loading, but they get full power when carrying a heavy load. To address that, they typically use a height-sensing proportioning valve at the rear. Those vehicles still have the two lines going to the rear, but they only go as far as that valve. From there, two other lines continue on to the wheels. That means there is actually four lines involved with the rear brakes.
If you need to replace an entire rear line and the line nuts are rusty, they are likely to round off. Always use a flare-nut wrench, (line wrench), to reduce that possibility, but a better alternative is to cut the line right by the nut, then tap on a six-point socket, and use a ratchet to loosen the nuts. Do not even bother trying to use a twelve point socket. If the nut is still too rusty for that, use a vise-grip pliers on the nut. Be careful with heat on those nuts. If you are trying to loosen the nut next to a rubber flex hose, using a propane torch usually will not get it hot enough. All it will do is make the hose's crimped connection mad. The hose will melt inside the crimp, then pop off with a loud bang and some smoke. That is not due to pressure in the line. It is from the pressure holding the hose in the crimped fitting. Propane is usually not hot enough to get a nut loose. You need an acetylene torch for that, but be aware that if you need to resort to that, there is a chance the nut will break in the middle and leave part of it inside the fitting where it cannot be removed.
Tuesday, June 20th, 2017 AT 11:25 PM