Double Flaring Tool

What Is a Double Flaring Tool — and Why Does It Matter?

When it comes to brake system service, few tasks are more safety-critical than forming a proper brake line connection. A double flaring tool is the specialized instrument that makes it possible to reshape the end of a metal brake line into the folded, reinforced geometry required by SAE standards. Getting this right isn't optional — it's the difference between a sealed hydraulic circuit and a brake fluid leak.

At the heart of every hydraulic brake system is a sealed circuit that operates under extreme pressure. Double flares — also known as inverted flares — are engineered specifically to handle this environment. As one brake system resource explains, brake flaring creates a metal-to-metal seal that holds under the 1,000 to 2,000+ PSI pressures common in hydraulic brake systems. A single flare simply cannot withstand these demands, which is why double and bubble flares are the only types rated for hydraulic brake circuits.

How a Double Flaring Tool Works

A double flaring tool uses a die block to clamp the tubing securely, then a two-step process first folds the tube end inward and then seats it flat — creating the characteristic folded-face geometry of a double (inverted) flare. The completed flare must be even all the way around with no cracks or gaps to ensure a reliable, leak-free seal. Most kits accommodate the most common brake line sizes including 3/16", 1/4", 5/16", 3/8", and 1/2" O.D. tubing in both SAE and metric (4.75mm DIN) standards, making them suitable for domestic and European vehicles alike.

There are several design formats to consider: traditional manual clamp-style tools are the most common, clamped in a vise and using dies with a threaded press. Lever-actuated and hydraulic designs are also available for shops that need faster throughput or need to work on tubing in tighter spaces.

When Do You Need a Double Flaring Tool?

Any time a brake line is cut, spliced, or replaced, a proper double flare must be formed on the tubing end. Common scenarios include:

• Repairing or replacing corroded brake lines, particularly in rust-prone climates
• Fabricating custom-length hard lines for a restoration or performance build
• Splicing a damaged section of line while keeping the rest of the system intact
• Replacing brake lines on classic or vintage vehicles that use standard 3/16" steel tubing

It's also worth noting that whenever the hydraulic brake circuit is opened, the brakes must be bled to remove any air introduced into the system before the vehicle is driven again.

Tips for Clean, Leak-Free Flares

Tool quality matters enormously in this application. A poorly formed flare is one of the most common causes of brake fluid leaks after a line replacement — even a hairline imperfection in the flare surface can allow fluid to weep under pressure, leading to a spongy pedal or even complete brake failure. Always use a tubing cutter (not a hacksaw) to ensure a clean, square cut, deburr the inside and outside of the tubing end, and apply a light lubricant before pressing the flare. Nickel-copper coated steel tubing (often called NiCopp) is widely regarded as the preferred material for brake line work, as it resists corrosion and is easier to flare than bare steel.

Top Brands in Double Flaring Tools

Imperial is a long-established name in professional tubing tools, offering a range of flaring kits designed for both bench and in-the-field use. Imperial's double flaring tools feature hardened steel dies, self-centering yoke designs, and hardened chrome-finished cones built to withstand the repeated pressure required to form consistent 45° double flares on steel, copper, and aluminum brake tubing — making them a dependable choice for mechanics and serious DIYers alike.