The Process Behind Flaring a Brake Line

Flaring a brake line isn’t as hard as it looks, but there are a few steps you really need to nail (ahem… line preparation!) that will make or break your flare. Learn to nail these steps and you’ll end up with a tight seal in your line. Don’t bother mastering them and you’ll end up with leaks that have your brake pedal sinking down to the floor when you’re finished swapping the lines. (The last thing you want when you’re done bleeding the lines and the master cylinder!)

A good double flare will help seal your brake line and give your system the strength it needs to stop again and again. A poor double flare will leak and keep your vehicle from braking properly. That’s why it’s so important to take the time to learn to do it right!

The following double flare brake line instructions will help you learn the whole process properly. Follow along and make sure you know what you’re doing before you try it yourself. This is not a repair you want to rush through because you’re going to spend more time installing your line than you are making it. Get the flares right or you’ll be wasting time pulling line back off again and again. And that my friend is how you take a one-day job and turn it into a week-long nightmare marathon!

Choosing the Line

The brake line you choose is pretty important. You have three main options to pick from, copper brake line, steel and stainless steel. Copper and stainless steel are about the same price, but copper is significantly easier to work with. It’s softer and will flare with lower-quality equipment. Stainless steel is the most durable but it’s brittle and very hard, meaning you pretty much need pneumatic equipment to flare the line properly. Steel brake line is about half the cost of the other two options, but it doesn’t last as long as they do. It’s harder than copper, but a decent quality flaring tool will handle it without an issue.

The tool pictured up above is the same one that I use to make my flares and it’s much nicer than the bar-clamp style tools. It’s an in-line flaring tool kit if you decide to pick one up at an auto-parts store. (Just be aware that not all stores carry them and they will probably try to hand you a bar-clamp style tool, don’t take it!) In fact, I went to rent a tool after not liking my bar-clamp tool and Autozone had a bar-clamp style for rent.

When using an in-line flaring tool kit like the one I own today the flaring process is more reliable. Instead of gripping the line with a bar clamp and then twisting a cone down to make the flare while hoping the damn thing doesn’t slip off or make an uneven flare, this guy gives nice even pressure on all sides at the same time.

I bought the bar clamp style tool first then tossed it out because the flares weren’t as consistent. The in-line flaring tool above is still on my shelf because you lock a surrounding tube in place and you drive force down through that onto the brake line end, giving the line no direction to slip off toward. It’s the only way a line flare tool should work in my opinion!

Through the use of the thick tube, force is nice and even. That way, when you twist your wrench to flare the brake line you know you’re going to get an even double flare as long as you prepared the line properly. We’ll get to line preparation in more depth down below. If there’s any section you really pay attention to, that’s the one that’s most important!

Getting the Tool for Stainless Steel Line

When I fixed my pickup I went with steel brake line after attempting stainless steel first. My brake flaring tools just couldn’t handle the hard brittle stainless line. I kept getting an uneven flare no matter how much prep I did.

With that said, you can do stainless steel brake line if you want, you’ll just have to spend three times as much on a flaring tool kit to get a professional pneumatic set. Like the one pictured up above. This is the highest rated kit on Amazon, but some select auto stores will have their own recommendations as well.

Pneumatic kits are the most effective to use overall, and they’re necessary for stainless steel. If you’re looking to make an affordable repair though steel or copper line and a basic flaring tool work just fine.

Preparing the Line

One of the most important steps to properly flaring a brake line is the preparation. If you don’t prepare the line properly it simply isn’t going to flare like it should. It’s vital that you take the time to chamfer the outside and inside of the line end. That means to cut away a tiny section of the line at a 45 degree angle to give it a bit of a beveled edge. To do this you should carefully grind away some of the line using a file or a dremel bit. During my brake line project I liked using my Dremel with a few bits to do this job.

This wheel grinder bit works well for chamfering the outside of the line, and this cone-shaped grinding bit works well for the inside of the line.

Spin up the wheel and move it around the outside of the line with it at an angle to create a nice chamfer. Make sure you take your time until you see the outside edge of the line start to round and lose its harshness. This really help make a nice even flare.

Stick the tip of the grinding cone right in the line and spin it at a medium speed to chamfer the inside and you’re done. This only takes a minute or so and you’ll notice your line edge gets slightly thinner looking when you’ve got it right.

If you decide to go with files this process will take a bit longer. You can do the outside with full-sized files, but you’ll need something smaller for the inside. I started with a set of tiny diamond files, but it’s a pain to do it this way.

If there are any double flare brake line instructions that you follow, it should be these prep steps. If you do them properly you’ll be in much better shape when it comes to actually flaring the lines.

With a propre chamfer in place you’ll notice that the flare goes more smoothly. You’ll need less pressure to complete the flare and you’ll end up with a more even flare.

Master proper prep before completing flares that you use on your vehicle for seals that last.

Put the Flare Nuts in Place

Now that you have your line prepped, it’s time to slide the flare nuts in place. Put two of them onto your line. Set them so the threads are facing out toward the end of the line, you want the threads facing in opposite directions outward. Make sure you put the nuts in place before you start flaring your line, or you could end up with a line you can’t attach!

Insert the Brake Line

With the flare nuts in place you can insert the brake line into your flaring tool. This step is the same no matter if you are using an in-line tool or one of the bar-style tools for flaring a brake line. Slide the brake line through the proper section of the flaring bar or the square flaring clamp depending on your kit. Your want the line sticking out the same amount as the ledge on your adapter.

When it’s sticking out the proper amount lock it in place using the clamps or the hex key adjustments on the side.

Completing the Flare

Apply a couple drops of brake fluid to the flaring cone and the little adapter of the proper size for your line diameter. Now set the small tip of the adapter into the brake line itself. The rest of the instructions depend on the kit that you’re using.

Bar and Clamp Flaring

With the adapter in place carefully lock the yoke assembly onto the bar of the tool and insert the bar so that you can twist it down to apply pressure. Twist the cone down onto your adapter to press it all the way down flush against the bar tool. Now loosen it up and remove the adapter from the brake line. Finish the double flare by twisting the lubricated cone down into the brake line itself just enough to flare it out.

Congrats you just completed your first double flare!

Don’t be discouraged if it’s a bit uneven, when you use the bar and clamp style units there is a chance of making a lopsided double flare. Practice several times and you’ll get the hang of it over time.

In-Line Flaring

Since you sprung for the in-line tool finishing your flare will be simple. Twist the large threaded fitting down over your square clamp with the adapter inserted. Now simply thread the cone down into the threaded fitting to apply pressure to the adapter down below. Fit a socket over the end of the cone piece to tighten it down all the way. Loosen the cone piece, unscrew the threaded fitting and remove the adapter. Put the fitting back in place and screw down the cone piece into the empty brake line to complete your flare. Once you feel pressure only go a single turn or two to create a nice flare.

Making double flare brake lines isn’t too difficult, but you have to follow all these steps to do it properly.

You’ll probably mess up the first few times and that’s okay. Take your time and burn through a bit of brake line until you feel like you have the process mastered. Brake lines are cheap, but you’ll waste a bunch of time installing lines that aren’t made properly. Once you’re confident about flaring a brake line you can replace lines as needed on your vehicle and save a bundle.