If you have a section of brake line that needs replacing, the joint between the existing line and new section must be secure. Due to the forces generated inside brake lines under pressure, a compression fitting is not a good choice for most on-road automotive applications. That's why creating a double-flared connection is your best, and safest, bet when working on brake lines. Flaring a brake line is not difficult as long as you have the proper tools. Below is a list of what you will need as well as a step-by-step procedure for double-flaring a brake line:
Tools and materials
Steel brake line in the appropriate size for your vehicle
Steel flared fittings for brake line
Flaring die matching the diameter of your brake line
New brake fluid
Brake cleaner spray
1. Cut the brake line to the appropriate length - The first step is to determine how long you wish to make the brake line and then cut it to a usable length. Once you have measured the gap that needs coverage with a new line, attach a tubing cutter to the brake line at the desired cutting location. Turn the clamp handle on the cutting wheel so that it applies just enough force to grasp the tubing but not so much that it digs into the tubing. Start rotating the cutting wheel around the tubing and slowly turn the clamp handle clockwise to gradually increase the cutting depth as you proceed. Continue this gradual process until the line is cleanly cut into two pieces.
It's important not to rush the cutting process, or you risk crushing the line. If you do dent or crush the line, even a small amount, then you will need to start over from the beginning in a new area. Brake lines must be perfectly round at the end to perform a reliable, secure flare.
2. Chamfer the end of the brake line - Once you have cut your brake line to the desired length, grasp it firmly about two-to-three inches from the end and use a flat file to taper the end of the line. Ideally, you will create a 60-degree taper from edge to tip when viewing the line in a vertical position. Be sure the taper is even all around the edge of the line and extends to the inside of the tube wall.
3. Prepare the brake line for flaring - When the end of the brake line is chamfered, you will need to ready the line for the flaring process. Insert the flaring bar into your bench vise and secure it so that it will not move when struck with a moderate amount of force. Insert the flared fitting over the end of the brake line--it won't fit once you have flared the tubing--and position the line so the working end is inserted through the bottom of the appropriate hole in the flaring bar. Adjust the line, so the tip is protruding above the base of the flaring bar by about ⅛-inch, then tighten the clamp on the flaring bar to hold the line in position. Use your file to flatten the end of the line and remove the sharp edge generated by the chamfering.
4. Ream the inside of the brake line - After flattening the end of the line, use a reaming tool to create a small flare inside the tip of the brake line. Apply a small amount of clean, unused brake fluid to the tip of the line to help lubricate the area and make the reaming easier. Never use oil on any product that is integral to an automotive braking system.
5. Flare the brake line - Once the preparatory steps are finished, you will be ready to flare the brake line. Add a few additional drops of brake fluid to the inside of the brake line opening, then insert the appropriate-sized flaring die into the end of the line. Position the yoke above the die, then slowly tighten the yoke clamp until it pushes the cone into the die. Continue tightening the cone while monitoring to be sure the die is pushing down squarely without any deviation. If you detect the die or line is slipping, then be sure to stop and reset the die and yoke as needed. Once the bottom face of the die is flush with the surface of the flaring bar, then you are finished and can remove the line.
6. Clean and inspect the finished flare - After everything is completed, spray the newly-flared end of the line with brake cleaner to remove any debris and brake fluid. Carefully examine the flare to ensure it is completely round with no bulging or other imperfections.
If you have any trouble with these instructions, contact an auto mechanic from a company like Big Mechanic for assistance.