How to Make a Double Flare Brake Line – Step-by-Step Guide

What Is a Double Flare and Why It Matters for Brake Lines

A double flare is the standard flare type required on brake lines in North American vehicles. Unlike a single flare, which folds the tubing end outward just once, a double flare folds the metal back on itself — creating two layers of material at the seating surface. This design dramatically increases surface contact with the fitting, reduces the chance of cracking under pressure, and meets the SAE J533 specification that most OEM brake systems are built to.

The short answer: to make a double flare brake line, you cut the tube square, deburr the end, insert a stepped punch (the "adapter" or "button") into the tube, compress it to create a partial fold, remove the adapter, then make the final compression to complete the double fold. The entire process takes under five minutes per fitting once you have the right flare tool and a bit of practice.

Getting this right is not just about following steps — it is about understanding what can go wrong at each stage. Brake line failure from a poorly made flare is a direct safety hazard. Federal Motor Vehicle Safety Standard 105 requires hydraulic brake systems to withstand pressures well above 1,000 psi during panic stops. A defective flare can split open at a fraction of that pressure. This is why the quality of the flare, and by extension the quality of the automotive brake valves and fittings in the hydraulic circuit, directly determines whether a brake system holds together when it matters most.

Tools and Materials You Need Before Starting

Working with brake tubing requires specific tools. Using the wrong ones — or improvising — almost always results in an unusable flare or a weak joint that will fail under pressure. Below is what you need before cutting a single piece of tube.

Double Flare Tool Kit

A quality double flare tool kit includes a clamp block (die), a yoke (the driving frame), a cone-shaped ram, and a stepped adapter button for each tube diameter you work with. Common kits cover 3/16", 1/4", 5/16", and 3/8" tubing. Avoid cheap kits with pot metal dies — they deform unevenly and produce oval-shaped flares. Brands like OTC, Mastercool, and Lisle make cast-iron or hardened steel dies that seat consistently across dozens of uses.

Tubing Cutter

Use a dedicated tubing cutter rather than a hacksaw. A hacksaw leaves burrs and rarely produces a perpendicular cut. A quality tubing cutter scores the metal cleanly around the circumference as you rotate it. For brake lines, a mini cutter with a small profile is easier to use in tight engine bays or under a chassis.

Deburring Tool

After cutting, the inside edge of the tube has an inward burr created by the cutter wheel. A triangular deburring blade removes this cleanly. Leaving the burr in place restricts fluid flow and can break off inside the system, contaminating the brake fluid and potentially jamming automotive brake valves or ABS modulators.

Correct Brake Line Tubing

Standard OEM brake line material in North America is SAE J1047-compliant double-wall steel tubing, sometimes referred to as "bundy tube." It has a copper-brazed inner layer and a zinc-coated outer layer. Do not use hydraulic line tubing, compression fittings, or fuel line — these are not rated for the combination of pressure and vibration that brake circuits experience. Most replacement brake lines sold in auto parts stores come pre-coated with a polymer layer (NiCopp or Cunifer are popular alternatives) to resist corrosion.

Thread-on Fittings

Always slide the correct fitting onto the tube before making the flare. This is the most common mistake beginners make — completing a perfect flare only to realize the nut cannot pass over it. Metric and SAE fittings are not interchangeable. Verify the thread pitch and seat angle (45° for double flare, 37° for AN/JIC flares) before purchasing replacement fittings.

Step-by-Step: How to Make a Double Flare Brake Line

Follow these steps in order. Skipping or rushing any stage is the primary reason DIY flares leak or crack.

Step 1 — Cut the Tube Square

Position the tubing cutter on the line and rotate it gradually, tightening the cutting wheel by about a quarter turn every two full rotations. Do not tighten aggressively — this deforms the tube and creates an oval cross-section that cannot seal properly. The cut is complete when the wheel passes through the wall cleanly. The end face should be perpendicular to the tube centerline within about 2°.

Step 2 — Deburr Inside and Outside

Insert the deburring blade into the tube end and rotate it several times until the inner edge is smooth to the touch. Then flip the blade and lightly chamfer the outer edge. Any raised metal at the tube end will prevent the adapter button from seating flush, resulting in an uneven first fold.

Step 3 — Slide the Fitting Nut onto the Tube

Thread the brass or steel brake line fitting nut onto the tube with the open (threaded) end facing toward the tube end you just cut. This is the step people forget. The nut must be on the tube before the flare is made — it cannot be installed afterward. Slide it back at least 4 inches from the end so it does not interfere with the clamp die.

Step 4 — Clamp the Tube in the Die Block

Open the clamp block and select the correct hole size for your tubing. Place the tube in the die so it protrudes above the face of the clamp by a specific amount — this protrusion distance is critical. Most flare tool dies have a step or ridge to reference the correct height. For 3/16" tubing, the tube end should sit approximately 1.5 to 2 mm above the die face. Tighten the clamp bolts evenly and firmly so the tube cannot rotate or back out during forming.

Step 5 — Insert the Adapter Button and Make the First Press

Take the stepped adapter button that matches your tube diameter and place its stepped end into the tube opening. The larger shoulder of the button rests on top of the die face — it is this shoulder that controls the fold depth. Position the yoke over the die, align the ram with the button, and slowly wind the ram down until it contacts the button shoulder and presses it firmly into the tube. Continue pressing until the yoke ram bottoms out or you feel solid resistance. This first press folds the tube end inward and downward, creating the inner bell of the double flare. Do not overtighten — two firm turns past initial resistance is generally enough.

Step 6 — Remove the Button and Make the Final Press

Back the ram off, remove the adapter button from the tube, and reposition the yoke with just the cone-shaped ram centered over the partially folded tube end. Slowly wind the cone down into the folded material. The cone presses the inner fold outward and downward against the die face, completing the second fold. The finished flare should look like a short, wide trumpet with two distinct layers of metal visible at the seating face when viewed from the side. The outer diameter of the finished double flare on 3/16" tubing is typically around 9 to 9.5 mm.

Step 7 — Inspect the Finished Flare

Remove the tube from the die and examine the flare closely. The seating surface should be smooth and concentric — meaning the flare is centered on the tube axis with no tilting. There should be no cracks in the metal, no splits at the fold line, and no wrinkles at the outer edge. Gently run a fingernail across the seating face: it should feel uniform in thickness all the way around. A flare with any cracks must be cut off and redone. Do not attempt to press a cracked flare further — work-hardened cracks in brake tubing do not close under further pressure.

Common Mistakes That Ruin a Double Flare

Most bad flares come from a small set of repeating errors. Knowing these in advance saves material and time.

• Wrong protrusion height. If the tube extends too far out of the die, the first press creates too much material to fold back cleanly and the flare cracks or folds unevenly. Too little protrusion results in a thin, shallow flare that cannot seal at adequate pressure.
• Skipping deburring. A burr on the inner edge prevents the adapter button from entering the tube cleanly, causing the first fold to start at an angle. This creates an asymmetric flare that leaks even when the fitting is properly tightened.
• Using the wrong adapter size. A 1/4" adapter button in a 3/16" tube will not enter far enough; a 3/16" button in a 1/4" tube will bottom out too deep. Either way the fold geometry is wrong.
• Over-tightening the clamp. Crushing the tube oval in the die block means the flare will be oval, not round. An oval flare cannot seal against a round fitting seat.
• Forgetting the fitting nut. This is both the most common and the most frustrating mistake. Once the flare is formed, the nut cannot go on. You must cut the flare off and start over.
• Using incompatible tubing material. Stainless steel brake tubing requires a different technique — it work-hardens quickly and often needs annealing (heating to red with a torch and allowing to air-cool) before it can be double-flared. Attempting to flare cold stainless without annealing almost always results in cracks at the fold.

How the Double Flare Connects to Automotive Brake Valves and the Hydraulic System

A double flare is only one connection in a larger hydraulic circuit. Understanding where that connection sits — and what it is feeding — helps you make better decisions about repair strategy and component selection.

Brake fluid from the master cylinder travels through hard lines (the tubing you are flaring) to several key components before reaching the calipers or wheel cylinders. These include automotive brake valves such as the proportioning valve, the combination valve, and on ABS-equipped vehicles, the hydraulic control unit (HCU). Each of these has inlet and outlet ports with threaded seats that accept double-flared line ends through brake fittings.

The proportioning valve, for example, reduces hydraulic pressure to the rear brakes relative to the front during hard stops — preventing rear wheel lockup before the fronts. The combination valve packages the proportioning valve, a metering valve (which delays front disc engagement to allow rear drum shoes to contact the drums), and a differential pressure switch into a single unit. All three functions depend on precise hydraulic pressure delivery, which requires leak-free connections at every flare joint feeding the valve body.

When you replace a section of brake line that connects to any of these automotive brake valves, the double flare you make at the valve port end is under the same pressures as the rest of the system — commonly between 600 and 1,200 psi during normal braking, with peaks above 2,000 psi during ABS activation. A flare with a hairline crack that initially seeps only a drop of fluid will fail completely as brake heat cycles the metal and the crack propagates. This is why visual inspection alone after making a flare is not enough — you should also perform a pressure test by bleeding the system and then pumping the brake pedal firmly 20 to 30 times while checking every new connection for weeping fluid.

Many late-model vehicles use pre-bent factory brake line assemblies that thread directly into the HCU or into integrated automotive brake valves mounted on the frame. When these lines corrode (which typically starts within 8 to 12 years on vehicles in salt-belt states), it is often the flared end nearest the valve that fails first — because that end sees the highest vibration from road inputs and the most thermal cycling from underhood heat. Replacing just the failed section requires making a new double flare that matches the original geometry exactly, since the fitting and port are already fixed.

Double Flare vs. Bubble Flare: Knowing Which One Your Vehicle Needs

European vehicles — particularly those from German manufacturers like BMW, Volkswagen, Mercedes-Benz, and Audi — frequently use a bubble flare (also called an ISO flare or DIN flare) rather than a double flare. The two look superficially similar but are not interchangeable.

Before buying a flare tool or tubing, confirm which flare type your vehicle uses by checking the factory service manual or by physically examining an existing fitting. The bubble flare fitting has a convex seat and a distinctly rounded tube end; the double flare fitting has a flat, angled seat and a multi-layered tube end. Forcing the wrong type into an automotive brake valve port or caliper banjo will strip the thread and crack the port — a much more expensive repair than the brake line itself.

Bending and Routing the Replacement Brake Line

Making a clean double flare is only part of the job when replacing a full brake line run. The new tube also needs to follow the original routing path closely to avoid contact with suspension components, exhaust pipes, and sharp chassis edges.

Using a Tube Bender

A quality ratcheting tube bender with the correct radius die for 3/16" or 1/4" tubing produces clean bends without kinking. Avoid bending by hand or around a round object — hand bends create flat spots that restrict flow and weaken the tube wall at the bend apex. The minimum recommended bend radius for 3/16" brake tubing is approximately 20 mm to the tube centerline. Tighter bends cause the outer wall to thin excessively.

Routing Considerations

Keep the new line away from exhaust components by at least 50 mm wherever possible — brake fluid boils at 205°C (DOT 3) to 260°C (DOT 5.1), but the tubing itself can weaken significantly at sustained temperatures above 150°C. Use all original mounting clips and grommets to keep the line from vibrating against the chassis. Unsecured brake lines that vibrate against metal surfaces will abrade through the outer coating and eventually crack the tube wall.

Planning the Flare End Sequence

When fabricating a new line with two flared ends, always make the more difficult or less accessible end first. If one end connects deep inside a wheel well and the other connects at the proportioning valve under the hood, flare the wheel-well end while the tube is still straight and easy to handle. Then route and bend the tube before making the final flare at the accessible end. This avoids the frustration of bending a line to shape and then struggling to hold it steady in the flare tool at an awkward angle.

Installing and Torquing the Brake Line Fittings

A correctly made double flare will still leak if the fitting is installed incorrectly. These installation details matter.

Thread Engagement and Anti-Seize

Start every fitting by hand to avoid cross-threading. Brake line fittings have fine threads (24 threads per inch on 3/8"-24 fittings) that cross-thread easily when forced at an angle. Once hand-tight, use a proper flare nut wrench — not an open-end wrench — to finish tightening. A standard open-end wrench contacts the fitting on only two flats and rounds the hex easily. A six-point flare nut wrench wraps around five of the six flats and gives much better grip. Do not use anti-seize on brake fittings: the lubrication changes the relationship between torque and clamping force and can result in under-clamping even when the fitting feels tight.

Torque Values

Most automotive service manuals specify torque values for brake line fittings by size. A common specification for a 3/8"-24 (3/16" tube) brake fitting is 10 to 15 N·m (88 to 133 in-lb). For 7/16"-24 (1/4" tube) fittings, the range is typically 14 to 18 N·m. Over-tightening deforms the flare seat and creates a new leak path; under-tightening leaves the flare loose enough to weep fluid over time, especially after thermal cycling.

Connections at Automotive Brake Valves

When connecting a replacement line to automotive brake valves — whether a standalone proportioning valve, an inline pressure differential switch, or a full ABS hydraulic control unit — clean the port threads with brake cleaner before installation. Old brake fluid residue on the threads prevents proper torque reading and can hide damaged threads. If the port threads feel rough or require excessive force to start, inspect them with a thread gauge before proceeding. A damaged port on an ABS HCU typically means the entire unit must be replaced, as the aluminum housings are not easily re-tapped to a larger size.

Bleeding the System After Brake Line Replacement

After installing a new section of brake line, air is present in the system and must be removed before the brakes will function correctly. Air is compressible; brake fluid is not — and any air pocket in the hydraulic circuit means pedal travel goes partly into compressing air rather than moving fluid and applying the brakes.

The standard bleeding sequence for a four-wheel brake system starts at the wheel farthest from the master cylinder and works toward the nearest. On a typical front-engine, rear-wheel-drive vehicle: right rear → left rear → right front → left front. Always check the master cylinder reservoir level before and during bleeding — allowing the reservoir to run dry introduces a large air pocket that can take significant additional bleeding to remove.

If the vehicle has an ABS system with automotive brake valves integrated into the HCU, air trapped inside the valve body may not bleed out through conventional manual bleeding. Many ABS systems require a scan tool capable of activating the ABS pump and solenoid valves in sequence to move trapped air out of the HCU into the hard lines where it can be pushed out through the bleeder screws. Check the factory service procedure for your specific vehicle before concluding the system is bled correctly based on pedal feel alone.

After bleeding, pump the brake pedal firmly 20 to 30 times with the vehicle stationary and engine off, then check every connection you made for any sign of seeping fluid. A drop of brake fluid at a fitting joint is not normal and indicates either a defective flare, an under-tightened fitting, or damaged threads. Do not drive the vehicle until all connections are dry.

When to Use Pre-Flared Lines Instead of Making Your Own

For common vehicles — many GM, Ford, Chrysler, and Toyota platforms produced in volume — pre-bent and pre-flared replacement brake lines are available from aftermarket suppliers. These lines match the OEM routing and come with fittings already installed. In these cases, making your own double flare may not be necessary unless the pre-made line is the wrong length or an intermediate fitting connection is needed.

However, for vehicles where pre-made lines are unavailable (older or rare vehicles, custom builds, heavy-duty trucks with non-standard frame lengths), making your own double flare from bulk tubing is the correct approach. Similarly, when replacing only a corroded section between two unions — rather than the full line — a shop-made line with two freshly made double flares is often the most practical solution.

Compression fittings sold at some auto parts stores as "brake line repair kits" are not a safe substitute for a properly made double flare. These fittings rely on biting into the outside of the tube rather than seating against a flared end, and they are not approved for brake line use under FMVSS 105. Several states in the US prohibit their use in brake circuits by name. The time investment in learning to make a proper double flare is worthwhile — both for safety and because the skill is applicable every time any brake line in any vehicle needs repair.