A “flaring and swaging tool kit” off the shelf is really two tools bolted into one case, and brake line work only ever uses one of them. The flaring half forms the cone or fold that seats against a fitting nut. The swaging half expands a tube end so a second piece of tube can slide inside it and get soldered or clamped in place. Brake systems never use that second joint, so if you’re buying a kit for brake work, the swaging attachments in the box are dead weight. What you actually need is a flaring tool that handles 3/16″, 1/4″, 5/16″ and 3/8″ tube, does a proper double flare, and can be trusted on stainless if that’s what’s on the car.
What swaging actually is, and why brakes don’t use it
Swaging expands the last inch or so of a tube’s diameter just enough that another same-size tube slides inside it. That makes a lap joint, one tube inside the other, and the joint only holds together once it’s soldered, brazed, or clamped with a band. It’s a normal way to build a copper line set for a mini-split, join sections of dryer duct, or splice exhaust tube where the pressure inside the pipe is close to atmospheric.
Brake fluid runs well over 1,000 psi under normal pedal pressure and spikes much higher during hard braking or an ABS event. A slip joint held together by solder has no business seeing that kind of pressure, and no vehicle manufacturer builds a brake line that way. Every joint in a hydraulic brake system is a mechanical flare, cone or bubble, drawn tight by a threaded nut onto a matching fitting seat. There’s no soldered or brazed joint anywhere in the system, so the swaging side of a combo kit never gets used on a car’s brakes. It’s there for the other half of the market the kit is sold to, mostly HVAC and refrigeration techs.
What the flaring side has to actually do
For brake work, the tube sizes that matter are 3/16″, 1/4″, 5/16″ and 3/8″ outside diameter, and each one pairs with a specific fitting thread: 3/16″ is 3/8-24, 1/4″ is 7/16-24, 5/16″ is 1/2-20, and 3/8″ is 9/16-18. Get the thread wrong for the tube and the fitting starts on fine, then binds before the flare seats, which feels like a cross-thread even though the threads are cut correctly.
Every domestic car built since the 1960s uses a double flare at 45° SAE on steel or copper-nickel tube. A single flare is one fold of the tube wall over the cone punch. A double flare folds the tube back on itself first, then forms the cone over that fold, so the seat is two layers of wall thick instead of one. That extra thickness is what keeps the flare from cracking after a few thousand pressure cycles, and it’s why a tool that only has a cone punch, no separate fold die, can’t produce a real double flare no matter how it’s clamped. European cars often use a bubble flare (DIN) on the same 3/16″ tube instead, a rounded shoulder rather than a cone, and a custom AN-fitting brake line on a race or off-road build uses 37° JIC instead of 45° SAE. None of these three overlap, so check which one the car in front of you actually needs before assuming SAE covers it.
Where a combo kit falls short
The flaring and swaging kits sold at most parts counters are built around the swaging use case first, since that’s usually the bigger part of the market, and the flaring dies get treated as an add-on. Two things tend to be missing as a result: a fold die for a real double flare, and a clamp rigid enough to hold the tube square through two separate strokes instead of one. Skip the fold die and the kit only ever makes single flares regardless of what the instructions call it. A weak clamp lets the tube shift slightly between the fold pass and the cone pass, and the finished flare comes out thicker on one side than the other. It can still thread onto the fitting and look fine on the bench. What actually happens is a slow weep at the flare nut once the car’s back together and the line is under real pressure, and by then you’re pulling the wheel back off to find it.
Stainless makes this worse. It work-hardens as it’s formed, faster than mild steel or copper-nickel, so any flex in the die or play in the clamp is more likely to crack the fold or throw the cone off-center on stainless than it would on softer tube. A kit built for swaging aluminum AC line doesn’t need to hold those tolerances, and it usually doesn’t.
What to buy instead
Skip the combo kit and buy a flaring tool built for brake tube specifically. The BrakeQuip BQ351 Brake Flare Tool Kit covers double, single and bubble flare at 45° SAE across 3/16″ through 3/8″, with a 3/16″ DIN die included for European fittings, and it runs stainless as well as steel with a drop of oil on the die. There’s no swaging attachment in the box because there’s no job on a brake line that needs one. That’s a smaller kit than the combo version, and it’s the one that actually does what a brake line asks of it.