Short answer: for SAE brake line, use double flare, the tube end is folded over on itself to double the wall thickness at the sealing surface, which is what a brake fitting is actually designed to clamp against. Single flare is a different, thinner shape used on other fittings entirely, and the two are not interchangeable.
What “double” actually means
An SAE 45° double flare isn’t just flared twice for luck, the process folds the very end of the tube back inside itself before forming the final 45° cone, so the surface that actually contacts the fitting is two layers of tube wall thick rather than one. That extra thickness is what lets a double flare hold up to the pressure spikes and vibration a brake line sees over years of use without cracking at the flare.
Where single flare shows up instead
A single flare, one 45° cone formed directly from the tube wall with no fold, is common on fuel and transmission cooler lines, where the pressures and duty cycle are gentler. It is not a substitute for a double flare on a hydraulic brake line, even though the two can look similar enough at a glance to fool someone flaring their first line. AN 37° and ISO/DIN bubble flares are different again, different angle, different shape, matched to their own fitting families rather than SAE double flare hardware.
How to tell which one you’re looking at
A double flare has a visible step or slight thickening right at the flared end, from the folded-over material. A single flare tapers smoothly from the tube’s normal wall thickness straight into the cone with no step. If you’re reworking an existing line and can’t tell which it is, don’t guess, a flaring tool built for double flare (like the ones below) forms the fold as part of the process, so using the right tool for SAE brake work makes the question moot.
Sizes and the fitting that has to match
Brake tube on most vehicles runs 3/16″, 1/4″, 5/16″ or 3/8″ outer diameter, and the flare nut’s thread scales with it: 3/8-24 for 3/16″ tube, 7/16-24 for 1/4″ tube, 1/2-20 for 5/16″ tube, and 9/16-18 for 3/8″ tube. None of that changes based on single versus double flare, the same nut threads onto either shape because the nut only cares about the outer cone angle, not what’s happening inside the tube wall. That’s exactly how a single flare ends up on a brake line by mistake: it threads on, it looks close enough, and the failure doesn’t show up until the line is under real pressure.
How the wrong flare actually fails
A single flare on a brake line usually doesn’t leak the day it’s installed. The thin single wall seats against the fitting’s cone well enough to hold at rest and even under normal driving pressure. It’s the pressure spike from a hard or panic stop, well over 1,000 psi at the caliper, more with ABS cycling, that a single layer of tube wall isn’t built to take over and over. What actually happens is the cone edge rolls or cracks under repeated flexing at that sealing surface, and what started as a barely-there weep after a few hundred stops turns into a line that lets go under the one stop that mattered.
Checking your own tool before you flare
Not every flaring tool marketed for brake work actually cuts a double flare. Some inexpensive kits, especially anything built primarily for AC or fuel line, only form a single or bubble flare even when the packaging says “universal”. Look at the die itself rather than the box: a real double-flare die works in two steps, a smaller pilot punch folds the tube end over first, then a larger cone punch forms the finished flare over that fold. A die with only one punch per tube size is making a single flare, no matter what the label claims.
Torque: past snug, not past sense
Both flare types fail the same two ways, too loose leaks right away, too tight cracks something and leaks later, so torque matters as much as flare type. As a general guide for double-flare brake tube fittings, 3/16″ tube runs about 10 to 15 ft-lb, 1/4″ tube 12 to 18 ft-lb, 5/16″ tube 20 to 25 ft-lb, and 3/8″ tube 25 to 35 ft-lb. A vehicle’s own service manual spec overrides any of this when the two disagree. Overtightening a correctly-made double flare can still crack it, and it’s rarely the first tightening that does it. It’s someone giving an already-right fitting one more quarter turn because it still looked like it was weeping.
What we stock
BrakeQuip Products carries flaring tools that form a proper SAE double flare, BQ351 and BQ350, plus AN 37° and DIN bubble-flare tooling in BQ352 for the fitting standards that call for it. See our step-by-step flaring guide for the full process.