How To: Make Brake Lines and Bubble Flares


Building custom cars for track days, or simply as restomods, takes a lot of fabricating. One of the things you’re most likely to make is a brake line. Modified cars don’t play by the rules; hence you might need to reroute the brakes or make new lines altogether.

It’s not a difficult job so long as you know how to do it and today we’ll show you how! We’ll also cover the parts you need as well as the materials necessary to complete this project.

Signs of a Bad Brake Line

Inspecting the brake lines is something that should be periodically done on any car, especially an older one. That being said, there are instances when a brake line simply fails. A failed brake line won’t instantly make itself known. Instead, you’ll have to hunt for small symptoms.

Sudden Brake Warning Light

If you suddenly get a brake warning light, that can mean one of two things – either your brake pads are gone, or your brake fluid level is low. Check the pads first. If they are worn, the brake fluid will be lower in the tank since your brake pistons have to extend further to grip the disc.

However, if the pads are fine and you’re still losing fluid, there’s a solid chance that your brake lines are shot somewhere.

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Brake Fluid Spots

What we’ve previously described leads us to one of the most obvious symptoms of a brake line failure – brake fluid stains on the ground. Park the car overnight and inspect the ground it’s parked on in the morning. If you find any fresh fluid spots, the chances are that you’ve found a broken brake line.

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Is Making Brake Lines at Home Safe?

This is probably the first thing that comes to anyone’s mind when someone talks about making brake lines at home. Is this a safe thing to do? Can you make solid, durable brake lines using nothing more than commonly available tools? The answer is yes.

It is absolutely possible to make brake lines that will be every bit as good as the factory ones. The key part is to do everything carefully following proper procedures. Brakes are your only means of stopping, so it pays to be vigilant when it comes to the quality of your work. Don’t cut corners or accept anything but perfect results, and you should be fine.

Fortunately for us, achieving perfect results isn’t that hard, given that you have the right tools at your side. Before we get to the guide portion of this article, let’s get familiarized with the anatomy of a brake line.

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Brake Lines, Double Flares, Bubble Flares and More

Brake lines come in the form of stainless steel or galvanized steel pipes. These run from the master brake cylinder or your ABS/ESP depending on your car. The majority of a brake line is solid. It’s only the ends of every brake line made of softer materials, accommodating for the wheel’s movement.

Those attachment points have fittings that allow you to screw on the flexible hose to the hard-line. Every brake line is flared at the end. Flaring is there to provide a better seal between the fittings. There are two types of flares generally used in the automotive industry – double flare and bubble flare.

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Double Flare

Double flare, or also known as the SAE inverted 45-degree flare, is the most common type of flare found on modern brake lines. It looks like a small funnel that’s been inserted into the line itself. The backside of the double flare is angled at 45 degrees, which is also one of the ways you can recognize it.

The double flare is what we’ll be making today for our build. It’s a pretty simple thing to make with the right tools.

Bubble Flare

Bubble flare or DIN flare is the other type of flaring used on brake lines. You’ll notice that bubble flare looks more like a top of a screw, with its 90 degrees flare at the bottom. Bubble flares are often mistaken for double flares, which can lead to some confusion. That being said, these two types of flares use different hardware and are not interchangeable.

How to Make Double Flare Lines – Step-by-Step guide

Now that we know the basics of brake lines let’s get to actually making some. We’ll need some tools for this project, including a mini tube cutter and a line flaring kit. You might want to rent a professional flaring kit as the cheaper ones often won’t cut it.

Aside from these tools, you’ll also need a brake line. We suggest using the copper-nickel alloy brake line as it is easier to work with than your standard stainless steel. On top of that, this alloy allows for much better double flares.

You can technically flare brake lines while they’re in the car. However, you might find it difficult unless you either have a helping hand or a small vise of some sort.

That being said, let’s get started with the tutorial section of this guide.

  • Step 1 – Cut the Lines  – The first thing you’ll want to do is measure your lines, and cut them down to length. To cut the lines, tighten down the line cutting tool at a location where you want to make the cut, spin the cutter around the line. Once it’s no longer offering resistance, tighten the tool some more and spin it around the line again. Repeat the process until the cut is complete.
  • Step 2 – Clean up the Cut – If there’s one thing that’s often overlooked when flaring brake lines, it’s cleaning up the cut. There will always be burs or uneven edges at the place where you’ve just used your cutting tool. Mount the line into a vise and file it until you’re left with a clean, smooth crown. Additionally, take a box cutter or a knife with a similar edge and run it around the inside of the line. Doing so will scrape off any debris.
  • Step 3 – Set Up the Brake Flaring Tool – With the line cut and prepped, set up your flaring tool inside a vise. Make sure only to grip one part of the tool since it’s made of two moving parts. Apply just enough pressure to keep the tool in place.
  • Step 4 – Apply the Hardware – Sliding the hardware on the line BEFORE flaring is probably one of the most common mistakes people make when flaring. Make sure that you have your hardware on those lines before moving forward. Otherwise, your perfect double flare is going to end up in the trash.
  • Step 5 – Clamp the Line with the Tool – The next step is to place the brake line into the adequate slot on the tool, ensuring that it is visually sitting at the same height as the insert you’ll use to flare the line. Once you’re sure it’s sitting even, clamp the tool down.
  • Step 5.5 – Lubricate the Line – Use brake fluid or brake fluid safe grease to lubricate the portion of the line that is about to be turned into a double flare. You don’t have to go overboard with the amount of lubricant. A little will do. Also, don’t use silicone-based lubricants as they are not safe to use on braking components.
  • Step 6 – Flare the Line – Insert the flaring insert into the brake line and install the guide tube over the insert making sure that it is seated properly. With the insert sitting firm in place, take the adequate wrench (in our case, it was 7/16″) and start tightening the flaring tool.

The next bit of info is important – when the flaring tool makes contact with the line, apply 1.5 turns, stop and take the tool out.

  • Step 7 – Double Flare It – The final step involves repeating the process. Leave the brake line clamped and install the guide tube back over the line. This time around, we’re not using the insert. Instead, just screw the flaring tool over the line and wrench it down until you feel resistance. At this point, execute ¾ of a turn and back off. That should give you a perfect 45-degree double flare.

Factory Brake Lines?

If you decide to get a set of factory OEM brake lines instead of making your own, you’re in the right spot. We offer a wide range of genuine replacement brake lines for various European vehicles. To find the ones that match your car, simply input your car’s details into our navigation tool, and it will show you a list of results.

In case you have any questions regarding our offer, any of the brake lines from our catalog, or whether they fit your car, feel free to reach out to us via email. Our customer support reps are standing by to offer whatever assistance you might need.

Posted in DIY How-To
4 thoughts on “How To: Make Brake Lines and Bubble Flares
  1. Adam

    That’s a very nice write up! I do have one thing to add. There should be a step 5.5 listed. Involving using a brake fluid safe grease for the flare, and pressing fixture. I mean just a little bit of Red Rubber Grease-or the like (made for braking systems), will leave the flare mark free-and smoother. The tool will last much longer as well. The same product is great for wheel cylinder, or caliper rebuilding. Keeps the parts from rusting. Not saying you can’t do it without the grease, but one should. I stumbled upon this product years ago, and never looked back!

  2. Victor Sabatino

    This write up and video was informative, but it definitely was not a demonstration of the way to make a “double bubble flare”. Actually, the flare that was demonstrated was a standard 45 degree double flare, not a bubble (also known as an ISO) flare.

    To my knowledge, there is no such thing as a “double” bubble flare. Making a bubble flare is a one step procedure that requires a forming die insert which is similar to a 45:degree double flare forming die, but with a differently shaped recess to cause a different result (which is rounded and has the appearance of a bubble, hence its name) on the end of the tubing.

    It is great to provide written and video information to help others grasp how to do something new, but to present a procedure and apply the wrong name to it just causes needless confusion for those who are trying to learn.

    As a side note, the shop tool (used in the video) which is typically attached to a bench and holds workpieces securely is a vise, not a vice. I see that erroneous word choice quite regularly. In the do-it-yourself community, it’s becoming nearly as common as the misuse of the words “then” and “than” are in the rest of the English-speaking (and i use that term loosely) world. Happy wrenching…

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