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How to Properly Flare Brake Lines and Why Not to Use Rusty Lines

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How to Properly Flare Brake Lines and Why Not to Use Rusty Lines

Created on: 2017-02-28

Check out this video showing you how to properly flare brake lines with the knowledge and expertise of the 1A Auto mechanics.

  1. step 1 :Flaring a Brake Line
    • Check that the line is not rusted
    • Cut the line with a pipe cutter
    • Smooth the inside of the line with a pick
    • Smooth the edge of the line with a file
    • Set the line into the line flaring jaws
    • Set the line depth with the adapter
    • Put the flaring clamp onto the jaws
    • Tighten the clamp into the line until it bottoms out
    • Check that the bubble is flat and even
    • Tighten the pointed end of the swivel into the line

Tools needed for replacement

  • General Tools



  • Specialty Tools

    Pipe Cutter

    Double Flare Kit

Installation Video
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Hi, I'm Mike from 1A Auto. We've been selling auto parts for over 30 years. We're dedicated to delivering quality auto parts, expert customer service, fast and free shipping, all backed by our 100% satisfaction guarantee. Visit us at, your trusted source for quality auto parts.

Brake lines are very important part of your vehicle's hydraulic brakes. They supply the pressurized fluid to either the wheel cylinders or the wheel calipers to help compress them and create that friction that slows your vehicle down. However, especially in our area, they tend to get really, really rusty. These can sometimes rot out so bad that the pressure blows through them or removing another part or just from the movement of the truck they can break right off like ours did here and cause your vehicle to lose brake pressure which is very unsafe.

In repairing a brake line, there are a few ways you can do this. You can either cut it, reflare it, and if it's close enough to another good piece of metal, you can put a coupling there, or you can go to two good pieces of metal, put fittings on either end with a new flare and put a new piece of line in between them, or in the case of ours where we really only had about a foot of good brake line, we're just going to replace the whole thing front to back. We're going to show you acceptable line to flare, unacceptable line to flare, and when you should start considering replacing an entire line as to making a patch.

Just as an example, we're going to show you what happens if you try to flare a line that's really bad, like this one is here. It’s not only the fact that the entire line is like this, so you wouldn't want to flare one end of it anyway because it's just going to rot out farther up. If you just had rust in a small section like this, it's perfectly acceptable to just cut this out and flare the good metal on either side of you have it. Just tighten down your pipe cutter. You don't want to use side cutters or a saw or anything to do this. You're just going to end up crushing it and collapsing it. Then you're not going to get a good flare anyway. You just make a couple of turns, tighten up the pipe cutter a little, make a couple of turns, tighten it up a little, and eventually it'll go through. Just be patient and get a clean cut because that's going to help a lot with the flaring process.

Once you get your cut, normally a pipe cutter will come with a little triangle file on it that you would just stick in here and swirl around a little to clean it up. Ours doesn't have that, so I'm going to use a pick to clean up the edges. Then a flat file just to make sure everything's nice and smooth.

This is a basic brake line flaring set. They do make more intricate hydraulic inline flaring units, but I haven't found a time when one of these won't get me by. You want to set it into the jaws in the correct size. This is quarter inch line, so it's going to go in the quarter inch clamp. We'll take the quarter inch adapter which sits in there. You can see that that is a step end. You want this wider step to be where you measure your line to. We'll just lay that on there until it sits flat and even with the end of our brake line. Flip the clamps over, tighten it down as tight as you can. If it's not super tight, it's just going to slide and you're not going to get a flare at all, let alone a good one. Set the adapter on there. Put the swivel on. I may have to back this off. Make sure that seats fully on there and that everything is nice and flat. Put the handle in and crimp that right down so it bottoms out.

Once it's bottomed out and flat on there, go ahead and back your tool off. You don't have to do this entirely. This right here is already a good example of why we don't flare rusty brake lines.

This is a good clean factory brake flare that's on there. This is called a double flare, so the first step is to make a little bubble, then you push the center in which is what we're after. Everything needs to be nice and smooth and symmetrical to work out right. Here you can see the rust has compromised the structure of the line. When I try to set it in, you have a whole bunch of bubble here, but nothing over here. The line is kind of twisted, it's not in there right. You would never use a flare like this. This isn't okay, but I'm going to go through with the next step and just show you how bad it really is so you can see the difference between using the good line and using something that's unsafe.

You would normally just take this, drive the cone in, get your nice flare on the end. We did not get a nice flare at all. That is why you can't use a rusty line for this. Here’s a good clean double flare, and a structurally compromised line from rust. It's crooked. It's not straight, it's not clean and smooth. That's not going to seal. It's not going to hold. Our brakes are just going to blow out again, the minute you try to use that.

We cut a nice clean piece of line that was on our old brake line. We set it up just the same way that we did on the other one. I just figured we'd save you the time looking at it and we're just going to flare it again just like we did with our other piece.

This is what you wanted that first step to look like. This is the bubble that gets you those nice, high edges so when we push down in the center, we'll get that cone, double flare shape. This is nice and even, nice and straight, nice and sturdy. It's not all laid off to the side and jagged and weak like the other one was. Now we'll just lay in the pointed part of our swivel, tighten that down.

There you have it: nice, clean, even, strong, just like the piece that was on there from the factory. This will seal up nicely and give us confidence when using our brakes. However, where our line is so rotted out, this one's not really worth saving. So we'll flare our new line, bend it up, and put it in the truck. Here's our flare on the new brake line. See, it's just like the flare we put on that decent piece of brake tube we had on the old line. There's really no difference as long as the tubing is structurally sound and you don't have it, if it looks like here and you can flare it, but it's all rusted out down here, just cut all that out just like we did with our line. That's exactly how you want a nice, new flare to look.

Thanks for watching. Visit us at for quality auto parts, fast and free shipping, and the best customer service in the industry.

Tools needed for replacement:

    General Tools

  • File
  • Pick

  • Specialty Tools

  • Pipe Cutter
  • Double Flare Kit

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