The best and easiest on-car tool we have used for double flaring brake lines. 

There are several great flaring tools on the market, but when it comes to double-flaring tubing on a vehicle, this compact, On-Vehicle Flaring Tool from Granatelli Motor Sports works great when space
is at a premium.
Three steps are required to form an inverted flare using Granatelli’s tool. First, the tubing is inserted
into the tool against a stop that sets the depth of the tubing. Next, the first flare is created using one
side of the flaring die. The die is then flipped around and reinstalled to create the inverted flare.
The result is a flaw less double inverted flare that can be made without removing the line from the vehicle, perfect for making repairs or modifying an existing brake line or for fabricating new lines without taking everything back apart