When you make or replace a brake line, the flare at the end seals the fitting - and there is more than one type. Using the wrong flare causes leaks. Here are the brake line flare types.
Double (SAE / inverted) flare
The double flare, common on domestic vehicles, folds the tube end over itself to create a strong double-wall seal. It is the traditional flare and needs a double-flaring tool to form correctly - a single flare alone will crack and leak; see brake hard lines.
Bubble (ISO / DIN) flare
The bubble flare, common on many imports and newer vehicles, forms a rounded bubble at the tube end that seats in the fitting. It requires a different die in the flaring tool. Using an SAE flare in an ISO fitting (or vice versa) will not seal properly.
Match the flare to the vehicle
The critical rule: match the flare type to what your vehicle uses. Mixing them causes a slow leak that drops the pedal; see fluid leak signs. Check your existing lines or service info before flaring.
Do it right
Use a quality flaring tool with the correct die, cut and deburr the tube cleanly, and torque the fittings properly. Bleed the system after. Keep the whole brake system sound with quality pads and hardware.