How to double flare brake lines

Know How Notes – How to Make a Double Flare Brake Line

I still remember the first time I had to put a double-flare on a hard steel line. My dad and I were doing a brake job on our 1987 Suburban. The rear hard line for the wheel cylinder had rusted up and the nut was stripped. We had no choice but to cut it off and make a new hard line. Having never done this before, we went to the parts store for some advice. They provided a double flaring tool and a piece of hard line to cut and bend to size. Cutting and bending the line was easy, but we had a heck of a time figuring out how to flare the line, as the kit did not come with instructions. That was 25 years ago, now I can make a double-flare in my sleep. It is not difficult, but if you have never done it before, it can seem like a bit of voodoo. There are several key points that you need to know before you get started, so get ready to learn how to make a double flare brake line.

Tools

There are three types of flaring tools: manual, leverage manual, and hydraulic. Most of the time you will be dealing with a manual tool, usually mounted in a vise.

Manual- This what you will be dealing with. These kits are inexpensive and readily available at any NAPA Auto Parts Store. These tools are clamped into a vise, and use dies and a threaded press to impart the flare.

 

This is the most common tool you will find for flaring lines, the manual clamp. You can use these under the car on in a vise, making them very versatile.

This is the most common tool you will find for flaring lines, the manual clamp. You can use these under the car on in a vise, making them very versatile.

There are two types of flares: 45-degree and 37-degree. 45-degree flares are the most common in automotive use. AN or Army-Navy fittings require 37-degree single flares. This 37-degree tool looks the same, but notice that it does not come with the dies.

 

For AN fitting, you need a single 37-degree flare. This kit looks like the double flaring kit, but does not have the dies and has a 37-degree head.

For AN fitting, you need a single 37-degree flare. This kit looks like the double flaring kit, but does not have the dies and has a 37-degree head.

Hydraulic flaring tools are generally only used in shops where a lot of line flares are made on a regular basis, the average DIY mechanic does not need one.

The leveraged manual flaring tool uses dies and a swing-arm to press the flares into the line much faster and easier than a basic manual kit. These often cost a few hundred dollars, and are excellent for the builder that needs to make many flared lines, such as when restoring a car.

 

This is a leveraged manual flaring tool. It uses quick-set dies and a lever-actuated ram to press the flaring heads into the line. You can make a double flare in a matter of seconds with these kits.

This is a leveraged manual flaring tool. It uses quick-set dies and a lever-actuated ram to press the flaring heads into the line. You can make a double flare in a matter of seconds with these kits.

How to Make a Double Flare

We will focus on how to use the manual flaring tool.

The first step is to cut the line. The end of the line must be clean and smooth. Some kits come with a tubing cutter, you need one regardless. Tubing cutters work best when the blade is tightened after every pass. You don’t want to load the brake line into the cutter and just crank down the cutting wheel.

 

Tighten the cutting head once every pass around the line for get a clean cut. Don't forget to clean out the inside of the line after cutting.

Tighten the cutting head once every pass around the line for get a clean cut. Don’t forget to clean out the inside of the line after cutting.

Before placing the tubing into the flaring tool, the fitting must be slid onto the line. Don’t forget this, as you can’t install the fitting once the line is flared.

 

Make sure that the fitting is on the line and facing the right direction before flaring the end.

Make sure that the fitting is on the line and facing the right direction before flaring the end.

The flaring tool base is easiest to manage when secured in a vise, but can be used without a vise if necessary. The hard line should be loaded into the corresponding hole and the wing-nut clamps tighten enough to hold the line. Each tool comes with a set of dies. Each die is specific for a line. The die must match the line you are flaring. The die has a step, this is the depth of the line for the flare. Once this is set, the wing-nuts can be fully tightened.

 

For this demonstration, we have secured the base in a vise. Note that the line is clamped into the base flush with the step on the die.

For this demonstration, we have secured the base in a vise. Note that the line is clamped into the base flush with the step on the die.

Next, the die is inserted into the line with the stub sliding into the line itself.

 

The stub drops inside the line to secure the die.

The stub drops inside the line to secure the die.

The press locks over the base and the over the die. The press is threaded down until the die touches the base. This is the bubble portion of the flare.

 

Next, the press slides over the base and positioned over the die. Then the handle is cranked down to bubble the line.

Next, the press slides over the base and positioned over the die. Then the handle is cranked down to bubble the line.

The press is released and the die removed. The press replaced over the line and threaded into the line once again. You only need to thread it until the press is tight, you don’t want to crank it down.

 

This is what you should see with the the die removed, a small bubble.

This is what you should see with the the die removed, a small bubble.

 

The press is once again positioned over the line and threaded down. You just need to go hand-tight, don't crank too hard on it or you can damage the flare.

The press is once again positioned over the line and threaded down. You just need to go hand-tight, don’t crank too hard on it or you can damage the flare.

The finished flare seats against the fitting and when installed, will provide the seal for the fluid.

 

The final flare seats in the fitting and ready for installation in the vehicle.

The final flare seats in the fitting and ready for installation in the vehicle.

Flaring lines is easy as long as you follow the instructions. Next time you need to replace a hard line on your vehicle, you can be proud that you made it yourself. Of course if you have any apprehensions, you can always check with your local NAPA Service Center for assistance. 

To learn more about NAPA AutoCare, visit www.NAPAAutoCare.com.

 

about author

Jefferson Bryant

A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.

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3 Comments

  • Dan Miller

    March 11, 2016 at 2:17 PM

    Reply

    Thanks for the tip. We were using the single 37 degree for a Ford braking system. No luck.
    Now I see why.
    D.M.

  • Pingback

    February 15, 2017 at 8:19 PM

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  • Russ

    December 20, 2017 at 6:03 PM

    Reply

    Never tried double flare,but will now. Doing my 1989 Comanche rear lines, hate those one length fits all, looks like poor craftsmanship. Thanks, Russ.

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