How to Flare a Brake Line: Bubble vs. Double Flare and Choosing a Brake Line Flaring Tool

A brake line is only as good as the flare on the end of it. A flare that's lopsided, cracked, or the wrong style can weep fluid under pressure, and that's not a risk worth taking on the one system that stops your vehicle. The good news is that flaring brake line is a skill any careful DIYer can learn with the right brake line flaring tool and a little practice.

In this guide you'll learn what a flare actually does, the difference between a bubble flare and a double flare, how to pick the right flaring tool, and a step-by-step process for making a clean, leak-free flare.

Why Brake Lines Need a Flare

Brake lines carry fluid at high pressure, and the tube nut at each connection can't seal against a plain, straight tube end. The flare is the shaped end of the tubing that gives the fitting something to clamp against. When you tighten the tube nut, the flare is squeezed against the seat in the fitting, port, or union, and that metal-to-metal seat is what holds the pressure.

Because the seat has to match the flare, the flare style isn't a preference. It has to match what your vehicle's fittings were designed for.

Bubble Flare vs. Double Flare: What's the Difference?

Most brake line questions come down to this one: which flare do I need?

Double flare (SAE 45-degree)

A double flare is made in two stages. The tubing end is first folded back on itself, then pressed again to form a 45-degree cone with a doubled-over wall. That double thickness makes the flare stronger and less likely to crack. Double flares are common on many domestic vehicles and are used with inverted flare fittings.

Bubble flare (ISO)

A bubble flare, sometimes called an ISO flare, forms a rounded, bubble-shaped end that seats in a matching rounded port. It's common on many European and some import and domestic applications. A bubble flare is not interchangeable with a 45-degree double flare, even though both are used on brake lines.

How to tell which one you have

Look at the existing flare and the fitting it seats into. A cone with a folded-back wall points to a double flare. A rounded, ball-like end points to a bubble flare. When in doubt, check your vehicle's service information or compare against the old line before you cut anything. And never try to force one style into the other's seat. A mismatched flare is a leak waiting to happen.

Choosing a Brake Line Flaring Tool

There are several kinds of flaring tools, and the right one depends on how often you'll use it and how much room you have to work.

• Basic flaring tool kit: A clamp-style bar with a yoke and a die set is the classic budget choice. It works well on the bench and can do a good job if you take your time. A flaring tool kit typically covers common brake line sizes such as 3/16", 1/4", 5/16", and 3/8".
• Hydraulic flaring tool: A hydraulic flaring tool uses hydraulic pressure instead of a hand-cranked screw, which makes producing consistent flares easier, especially on harder tubing or when you're flaring several lines in a row.
• On-vehicle flaring tools: Some tools clamp directly to a line that's still on the vehicle, which helps when you're repairing a line in a tight spot.

Whichever you choose, make sure it makes the flare style you need (bubble, double, or both, depending on the kit), and that the dies match your tubing size. Pair it with a quality tubing cutter and a deburring tool, since a clean, square cut is the foundation of a good flare.

What You'll Need

• Brake line tubing sized for your application
• Tube nuts (installed on the tubing before you flare)
• A brake line flaring tool that makes the correct flare style
• A tubing cutter and deburring/chamfering tool
• A flare nut (line) wrench for tightening the connections
• Clean rags, brake cleaner, and safety glasses

How to Flare a Brake Line, Step by Step

1. Cut the tubing square. Use a tubing cutter rather than a hacksaw. A crooked cut leads to an uneven flare.
2. Deburr inside and out. Remove burrs and sharp edges so they don't tear the flare or contaminate the system.
3. Slide the tube nut on first. Make sure the threads face the end you're about to flare. It's easy to forget, and you can't add the nut once the flare is made.
4. Clamp the tube at the correct height. Follow your tool's instructions for how far the tubing should stick out of the die block. Too little makes a weak flare, and too much can crack or mushroom out.
5. Form the flare. For a double flare, use the first-stage adapter to fold the tube end, then the second-stage punch to finish the 45-degree cone. For a bubble flare, use the bubble flare die and punch your kit specifies. Tighten gradually and don't over-torque the handle.
6. Inspect the result. Look for an even, concentric flare with no splits, cracks, or flat spots. Scrap it and start over if anything looks off. The flare is cheap to remake.
7. Install and tighten. Thread the tube nut in by hand first to avoid cross-threading, then snug it with a flare nut wrench. Flare nut wrenches grip more of the nut than a standard open-end wrench, which helps prevent rounded corners.
8. Bleed and check for leaks. Bleed the brakes, have a helper hold firm pedal pressure, and inspect every connection for seepage.

Common Flaring Mistakes to Avoid

• Forgetting the tube nut. The classic mistake. Slide it on before flaring.
• Mixing flare styles. A bubble flare in a double-flare seat (or the reverse) won't seal properly.
• Skipping the deburr. Rough edges cause cracked or uneven flares.
• Practicing on the real line. Make a few test flares on scrap tubing first so you know how much tube to leave sticking out.
• Using the wrong tubing. Use tubing intended for brake systems, and if you're unsure whether a material is right for your application, check before you buy.
• Using compression fittings on brake lines. Compression fittings aren't a substitute for a proper flared connection on a high-pressure brake line.

Ready to Get Started?

If you're rebuilding a line, browse 4Lifetime's brake line tubing and fittings so the tubing, tube nuts, and flare style all match from the start. And if you'd rather skip the flaring altogether, a pre-bent complete brake line kit for your exact vehicle comes ready to install.

For the tools, check out our flaring tools and line wrenches to make the job cleaner and easier.

FAQ

What is a bubble flare used for?

A bubble flare is used on brake systems designed for a rounded ISO-style seat, which is common on many European vehicles and some others. It's formed differently from a 45-degree double flare, and the two styles don't interchange.

What's the difference between a bubble flare and a double flare?

A double flare folds the tubing back on itself and forms a 45-degree cone. A bubble flare forms a rounded, bubble-shaped end. Both are used on brake lines, but they seat in different fittings, so you need to match the style your vehicle uses.

Do I need a hydraulic flaring tool?

Not necessarily. A standard flaring tool kit can make good flares if you work carefully. A hydraulic flaring tool can make the job easier and more repeatable, especially if you flare lines often or work with tougher tubing.

Can I flare a brake line while it's still on the vehicle?

Sometimes. On-vehicle flaring tools are designed for tight spots, but a bench flare is usually easier to get right. If you can remove the line, flaring it on the bench is a good idea.