How to double flare brake lines

DISCLAIMER: This is one of those operations where if you screw up, you could kill yourself or others around you. If you are not 1000% sure of your ability to do the job, leave it to a professional!

There have been a few questions lately about repairing hard brake lines. I had to make a repair to the rear brake line on my Jeep today (yeah, Jeep, get over it ) so I took the opportunity to shoot some pics of the process.

The correct way to make or repair brake lines is to use double flaring. A single flare is simply flaring the end of the tube. A double flare actually folds the wall of the tube back over itself giving a very strong, compliant seal essential for brake lines. Single flaring results in a thin pipe wall at the base of the flare where the brake line will work harden and break over time. A single flare also will not conform to the inverted flare inside the fitting properly which causes it to leak. The double wall on a double flare compresses and conforms to the inverted flare and crushes to create the seal. Because of this, a double flare joint is considered to be a single use joint. However, people do get several disassembly/reassembly cycles out of them. This limited life cycle is why converting to AN fittings on a truck that gets taken apart a lot is cost effective. The seal design of AN fittings is intended to be taken apart many times with no loss of sealing effectiveness.

Making a double flare end on a brake line is actually very easy, but does require some practice to get right. It is common to make the flare off center or cocked to one side. Practice on scrap pipe until you get the hang of it.

It is crucial to have a decent double flaring set to make good double flares. This kit is from Harbor Freight and is marginal for making good flares. It’ll get the job done if you’re very patient and pay strict attention to detail.


This is the repair piece I made to put into the truck. The original pipe broke where the rear flex line attached. I bought a repair section from Napa, bent it to match, cut it to length, and reflared the cut end.


This is my Mighty Mite tubing cutter, available almost everywhere tools are sold. This is a super little cutter and easily gets into tight places. To use the cutter, slide the cutter over the pipe and position the blade where you want to make the cut. Tighten the knob until the blade just touches, then snug it slightly. Twist the cutter one or two turns and tighten the knob again. Repeat until the cut is complete. Don’t over tighten the knob or you’ll crush the pipe and have to start over. As you may have guessed, I flared this pipe before installing the threaded fitting :doh0715:


Here is the finished cut. The tubing cutter collapses the outer edge of the tube slightly. Some people remove the burr to open up the pipe again, some don’t. I don’t. The double flaring operation opens the pipe again to the correct diameter, so it’s not worth getting chips and filings inside the pipe.


After installing the threaded fitting onto the pipe, place the cut pipe into the flaring frame. The flaring frame is marked for different pipe sizes. It is cut with a countersink on one side, be sure the cut end of the pipe sticks out on that side. Select the correct flaring anvil from the kit. The anvils are also marked for pipe size. Most brake line is 3/16. Using the anvil, make sure the pipe sticks out of the flaring frame the width of the anvil flange. If it doesn’t stick out far enough you won’t have enough metal to get the double flare. If it sticks out too far, you’ll get a distorted bell and possibly too small an inside diameter for the finished flare. Tighten the flaring frame nuts securely. If they are loose at all, the pipe will simply push out of the frame when you start making the flare.


Place the tip of the anvil into the end of the pipe. Using the flaring press, force the anvil into the pipe until the anvil sits flush against the flaring frame. This forms the button on the end of the line. The button is formed by the countersink in the flaring frame and the countersink in the anvil.


This is the finished button. For some European cars, this is the finished flare, but not for our Broncos. Loosen the press and remove the anvil from the pipe. Do not remove the pipe from the frame yet. Center the taper on the flaring press into the center of the pipe and tighten the press securely. This collapses the upper bell on the button into the inside of the flare.


With the flare done, remove the pipe from the flaring frame and inspect it to make sure it’s square and centered relative to the pipe. If the flare is not square and centered, cut it off and start over.


Here is the finished repair. Inspect all joints for leaks after bleeding. The double flare must be collapsed to seal the joint, so it may take a few rounds of tightening to eliminate all leaks.


And that’s all there is to it! If you’ve practiced your technique, taken your time, and been very anal about the details, you will have a flared brake line joint that’s as good as the factory’s. :thumbup