Brake fluid leaking

Sounds like you're describing a leaking steel brake line that runs along the rear axle housing. Those are commonly covered with a long wound-up wire that looks like a spring, to protect the line from rocks and other debris. That wire also does a real good job of holding salt, dirt, and water next to the line.

This is a perfect time to warn against pushing the brake pedal more than half way to the floor, if you haven't done so already. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel inside the brake master cylinder. When you run the brake pedal all the way to the floor, whether surprised by a sudden leak, or when pedal-bleeding with an inexperienced helper, the rubber lip seals run over that crud, which can rip them. Most commonly that results in a slowly-sinking brake pedal, and that often takes two or three days to show up. That damage requires replacing the master cylinder.

At a minimum, the leaking steel line has to be replaced. Your best bet is to buy one that is already made, of the correct length or just a little longer. The easiest way to get the old one off, if it's very rusty, is to cut it right next to the line nuts, then use a six-point socket and ratchet to remove the nuts. Those nuts are made of very soft metal, so a regular box or open-end wrench will round them off, but not loosen them. Once that occurs, you might get them loose with a vise-grip pliers. The nuts on the new line should be tightened with a "flare-nut" wrench, aka, "line wrench". Those grip the nut on four corners instead of just two.

That new line is going to have to have the air bled out. They way I'd start is by tightening the nut at the three-way connector on top of the axle's center section, then loosen one corner of the cap on the brake fluid reservoir. That will prevent a vacuum from building up in there, which will let gravity pull the brake fluid down through the new line. Leave the nut at the wheel cylinder loose so the air can be expelled. When brake fluid starts dribbling out, snug that nut. If the brake pedal seems high and normal, you're done. Ideally we would open the bleeder screw on the wheel cylinder, but if the metal line rusted out, chances are that bleeder screw is rusted tight and will snap off if you try to open it.

If you need to bleed the wheel cylinder but you can't get the bleeder screw loose, your best bet is to replace the wheel cylinder. They're uncommonly inexpensive and will make bleeding much less frustrating. There are kits for drilling out and replacing a broken bleeder screw, but the cost of parts and the time involved make that a real bad option compared the cost of the new wheel cylinder.

Here's links to some related articles, if you need them:

https://www.2carpros.com/articles/how-to-replace-a-wheel-cylinder

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

These are extremely common items at every auto parts store. That's probably why they don't bother to advertise them on their web sites. You can also buy a whole roll of line that has to be cut to length, then you slide on the soft metal nuts, then form the double flares on each end. The double-flaring tool is a specialty tool, and not horribly expensive, but it's way over-kill for the one time you'd use it. Also, my experience is since that bulk steel line has been bent into a coil, once you straighten it out, the double flares always end up off-center. I don't trust them to seal properly. Also, as a point of interest, once the flare is formed, the last step is to bend the tube down with the tool. Never crush it as tight as you can get it. Just bend it enough so it seats on the fitting it's getting attached to. When you tighten the nut, the flare will finish squeezing itself tight. And it will have some space to bend so it matches any irregularities in the seat.

The pre-manufactured lines already have perfect double-flares that are guaranteed to seal. These lines will be in a display box with common standard lengths to pick from. Typical lengths are 12", 18", 24", 36" 48" and 60". Obviously you don't want one that is an inch or two too short. If, for example, you need a line that is 51" long, you could use the 60" line and put some S-bends in it to take up the extra length. Form those bends so there's no high spot where air could pool and be hard to bleed out. You could also buy a 36" and an 18" line, and use a union to couple them together.

Almost all brake lines are a standard 3/16"-diameter, but around this time, some manufacturers started using different styles of flares. You must use a line that has the same style of flare. To add to the frustration, some manufacturers, Ford in particular, use a double-flare on one end and a metric "iso-flare" on the other end. The auto parts stores will have those special lines in stock too. If you need a union to couple two lines together, that will also be listed as a 3/16" fitting.

The best approach is to remove the old line and measure the length it needs to be. Take at least the two ends to the auto parts store so the salesman can match it up to the new line. Your new line is not going to have that wire wrapped around it. That was done by the supplier that sold that line to the car manufacturer. Without that wire, it's pretty likely the new line isn't going to be held snugly by the metal tabs on the axle housing. All you're interested in is the new line has to be held solidly so it won't slide back and forth, which could cause a hole to wear into it, and it has to be supported so it doesn't vibrate. Vibration from road bumps will cause the metal to "work harden", meaning it will become brittle, then crack at a stress point, typically near one of the line nuts. I use nylon tie straps because they fit every occasion, and they're fast and easy.

Since my daily drivers are old, rusty vehicles, I replace a brake line about once every two or three years. I just bought a set of six pre-manufactured lines at an old car show swap meet for $9.00, but at an auto parts store, expect to pay around five to ten dollars for one piece. They also have something new within the last few years. That is the same line with an anodized coating to resist rust. Those would be a good value for a relatively new vehicle that is perhaps being repaired after a crash. For old rusty vehicles, a standard line is going to outlast the car, so that added expense of the anodized coating isn't necessary.

As long as you have the two ends, and you know the length, any parts store employee will find the right line. Often that display I mentioned is tucked away out-of-sight. You can also find these lines at any farm and home store and at a lot of hardware stores.

Only if you live down south. If the line rusted out, you can be sure the line nuts will be rusted to the line. When you try to turn the nuts, they're going to twist the line until it snaps. Also, those nuts are made of very soft metal. Regular wrenches will round the nuts off, adding to the frustration. That's why I started out right away by saying to just cut the line next to each nut, then use a ratchet and 1/4"-drive socket to remove the nuts. You'll probably have to tap the socket on to get it to slide over the rust. The most common size socket will be a 3/8", but some applications use a 7/16" socket.

By doing it this way, it should take less than five minutes to remove both nuts, then you'll have three pieces to take to the auto parts store. They'll be able to chose the best new line for you. There will definitely be one that's is long enough so you won't have to join two lines with a union.

There's no point in trying to free up the nuts and get them to spin freely because the line is going into the garbage anyway. Some people try using penetrating oil, but that absolutely must be avoided. Brake fluid must never be contaminated with the slightest hint of petroleum product because that will soften and grow the seals and other rubber parts. The only proper repair for that is to remove all parts containing rubber parts that contact the brake fluid, flush and dry the steel lines, then install all new parts with rubber in them. That repair cost can exceed the value of the vehicle, and it's much higher if there is an anti-lock brake hydraulic controller that also has to be replaced. These contaminants include engine oil, transmission fluid, power steering fluid, and axle grease. Brake system specialists even wash their hands with soap and water to avoid getting fingerprint grease on rubber brake system parts.

Also be sure to use no grease or oil on the new line nuts. If you want to try to prevent the nuts from rusting to the new steel line, at most you can coat them with spray paint, since that won't find its way into the brake fluid.

Most mechanics don't bother with a bending tool unless they do this job real often. It takes a little patience, but I use my thumbs to make small bends.

Be aware most salesmen at auto parts stores were never mechanics. They never did most of the jobs they're advising you on. They specialize in part numbers and applications.

I'm in Wisconsin, where they throw a pound of salt onto an ounce of snow. Rust is a part of our daily lives thanks to our politicians and highway "experts". A few years ago I was visiting one of my favorite "pull-your-own-parts" salvage yards in Nashville. While gathering up a bunch of spare parts to keep on hand, I figured I'd grab a rear brake assembly and spindle for my '88 Grand Caravan. To my amazement, the line nut popped loose with an open-end wrench, (all I had with me). That's when I really started appreciating what you don't have to put up with.

I assumed you were in a northern state since the brake line rusted out. Happy to hear you're not, and happy to hear you solved your problem.

Now, ... How are you fixed for '88 Grand Caravans with 15" wheels running around by you? I'd love to head south to find a rust-free one. Mine is so rusty, the carpet is the only thing holding the front and rear together!

Just to toss another option into the ring. Because GM had issues with the brake lines failing you can buy complete preformed kits for them at many parts stores, they also sell the individual lines as well. I've used a few of them and have no problems so far on any of the ones I did. The nice thing is that they offer the lines in steel, NiCop, and for many stainless.
The latter 2 won't rust out again. Inline tube makes some and you can also buy the same parts with the Dorman name on them but usually cheaper. They show a kit for your truck for about $80.00 in stainless.