Passenger car braking systems operate using hydraulic pressure. A master cylinder connected to the brake pedal sends fluid through metal lines to the wheel cylinders, which operate the brakes. The metal lines between the cylinders are connected with flared fittings.
Although easier to make, single flares are prone to splitting, galling and deformation, especially when serviced multiple times. For that reason, single flares are not generally used on brake lines, with the exception of the so-called Army Navy or AN-type 37-degree single flare fittings. The first step of a double flare, called a “bubble” flare, is strictly speaking a single flare, and is also used for brake lines, mainly on British cars.