Every steel, NiCopp® or stainless brake line seals at its ends with a flare: the tube end is formed into a shape that the tube nut presses tight against the seat inside the fitting. Use the wrong flare and the line will leak no matter how hard you tighten it. Almost every passenger car and light truck uses one of two styles.
The two common flare types
| SAE 45° inverted (double) flare | ISO / DIN bubble flare | |
|---|---|---|
| What it looks like | A 45° cone. The tube wall is folded back on itself, so you can see a doubled lip at the end. | A short, rounded "bubble" or mushroom just behind the tube end. |
| Tube nut threads | Inch (UNF) threads such as 3/8-24 on domestic vehicles; many Asian vehicles use metric M10 x 1.0 nuts with a double flare | Metric threads, e.g. M10 x 1.0 or M12 x 1.0 |
| Where it's common | Most domestic vehicles and many Asian vehicles | Most European vehicles and many newer GM and import applications |
Why not a single flare?
A single 45° flare is one layer of metal formed into a cone. It's fine for some low-pressure uses, but brake systems need the strength of a double flare (SAE) or a properly formed bubble flare (ISO). Don't use a single flare on a brake line.
How to identify what you have
- Look at the old line's end. A doubled, cone-shaped lip means inverted double flare. A rounded bulge set back from the end means bubble flare.
- Check the tube nut threads. Measure the thread diameter and pitch (see the thread size chart). Thread size alone won't tell you the flare type (many Asian vehicles pair metric nuts with a double flare), so always confirm with step 1.
- Measure the tube. 3/16" (4.75 mm) is the most common brake line size. 1/4" and 6 mm lines are also common.
Making a flare that seals
- Cut square. Use a tubing cutter, not a saw, so the end is square and clean.
- Ream and deburr. Remove burrs inside and out. A burr can split the flare or keep it from seating.
- Put the tube nut on first. Slide the tube nut on, threads facing the end, before you flare.
- Form the flare with the correct die for your tube size and flare type, following your tool's instructions.
- Inspect it. The flare should be centered, smooth and free of cracks. If it's off-center or split, cut it off and start again.
AGS tools for the job
- FF-3 Flare Force® Hydraulic Flaring Tool makes ISO/DIN bubble flares, 45° single and double inverted flares, and push-connect fuel flares on 3/16", 1/4", 5/16", 3/8" and 1/2" plus metric tubing. Sell sheet · Instructions
- FF-2 / FF-237 Professional Turret Flaring Tool. Instructions
- TC-316 ratcheting tubing cutter, TR-1 reaming and deburring tool, and TB-1 / TB-2 benders. Flaring tools & kits brochure
Need a ready-to-install option? Pre-flared AGS lines and EZ-Fit® kits come with the correct flare and tube nuts already installed. Shop flared & fitted lines.
Sources: AGS C-2531 Flaring Tools & Kits, C-2400 FF-3 sell sheet, C-2690 Fitting Guide.