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.
A double flare is accomplished using a special tool that first forms a "bubble" on the end of the metal brake line tubing, then flattens the bubble in on itself. The double thickness of metal reduces the problems associated with a single flare. This is the type of flare recommended by the Society of Automotive Engineers. A 45-degree double flare is used by the majority of American automakers.
Plain single flare connections are not recommended for use on brake lines. Double flare brake line connections are by far the most common. Properly made and cared for, they are sufficiently robust for most applications. Double flares can withstand several repair cycles before they should be replaced. Bubble flares work well but require frequent replacement. AN single flare fittings are part of a system designed to meet military specifications. They are by far the most robust type of brake line connections and made to endure frequent repair cycles. AN connections are the choice of the military, hot rod builders and race car mechanics.