Monday, December 29, 2014

How to repair a broken brake line with a double flare union

     When replacing a brake hose that was acting as a check valve, I encountered a problem. When I attempted to turn the new hose into a seized flared fitting, I found the keyed hose and it's mounting bracket were about 180 degrees out of bolt-on position. A properly freed flared nut is designed to turn into the brake hose with the hose held by the bracket. I chose to continually try to work the fitting loose. A better option here might be to drain the brake fluid completely out of the line and then heat the fitting with a torch. Brake fluid is flammable, so do not attempt this with brake fluid in the line. That being said, I chose instead to soak the flare nut and end of the line in a penetrant and try to free it up. In the end, I cracked the 3/16" line by holding it with a small channel lock pliers while trying to turn it with a 12mm flare nut wrench. This post covers repairing the broken line with a double flare union. I flared the broken end of the line on the car as well as a replacement piece of line I had to bend to match the original. I understand the double flare fitting is the only legal way to splice a line. The job requires a flaring tool kit. I used an Advance Auto loaner kit.

                                              Job at a glance

  • Safety:Gloves, safety glasses
  • Tools: Brake flaring kit, reamer or file, tubing bender, protractor, hack saw.  
  •  Parts and materials: Replacement brake line with correct flare fitting.Flare nut. brake fluid.
  •  Cost of parts and materials: Brake line $6, flare nut $2, double flare union $7.
  •  Shop labor cost for the job: ??? 
  •  Home mechanic estimated time: 3-4 hours   
     Flaring the line on the vehicle

If you haven't flared a brake line before, I recommend practicing on some scrap line. With some practice you should get good results. The most difficult flare is the one on the vehicle. I did that one first.

1) Cut the brake line in a place that is fairly straight. and accessible. I found this to be about eight inches from the end of the brake hose. I used a hack saw as there wasn't room to get a tubing cutter in there. This line had a protective steel coil around it which I also cut off. In addition I removed part of the coil a few inches further in to allow room for the flare nut to slip over the old line. I used an oscillating tool with a cut off wheel.

2) Preparation for flaring is a three step process. Proper preparation is critical in achieving a good result. Step one is squaring off the end of the tubing. Put the tubing in the flat end of the flaring bar and file it flush with the surface. Step two is to ream out the inside of the tubing with  a reamer, a knife or a small round file. Step three is to bevel the outside edge of the tube with a file.

Step one is squaring off the end of the line with a file


3) Slip the flare nut on the old brake line. I had to straighten the line a bit to get the nut on far enough.

4) Now we're ready to flare the brake line. Fit the tubing to be flared in the concave part of the flaring bar. Set the height of the tubing above the bar surface at the height of the first step of the button-like anvil.

5) Cinch the flaring bar down tightly and evenly on the tubing. Check the height again with the anvil. If the tubing is not held tightly enough in the bar, it may be pushed through, ruining any chance of a good flare.

6) With the little nipple on the anvil in the tubing, carefully center the driving tool, conical point over the anvil and tubing. Put a drop of brake fluid on the driving surface of the anvil. It must drive on straight. I had just enough room to turn the sliding handle of the driver around to be able to bottom out the anvil.

With everything aligned and centered, press the anvil into the tubing


7) When the anvil is bottomed out, back off the driver and remove the anvil. Inspect the flare. It should be even all the way around. Inspect the pressed end of the tubing. It will be "bubble“ shaped. If it looks even and centered proceed to the next step.

Look for an even bubble shape after pressing the anvil into the tubing


8) Using just the conical driver now, center it in the tubing and drive it down until the tubing flares out to it's limit. Inspect. The flare must be even, centered and complete. If it looks good, the worst is over. If it doesn't, repeat steps one through eight again.

This is a good flare


Bending a replacement piece of line

     I needed about a ten inch piece of brake tubing to go from the brake hose to my newly flared tubing. I used the original piece of now broken off line as a template to make a replacement. The original had three bends in just that short stretch. While I was able to duplicate them pretty well, the important thing is just to get to the other fitting. This procedure is for duplicating the bends.

1) Measure the overall length needed for the replacement tubing. I used a cloth measuring tape of the kind used in sewing.

2) Using a tubing cutter, cut the new piece to that length.

3) To duplicate the angle we need to know two things. 1) The distance from the end of the tubing to the center of the first bend and 2) the angle of the bend. Measure the length. Mine was 7 cm. Use a protractor to find the angle of the first bend from the brake hose end. Lay the straight part parallel on the straight edge of the  protractor with the center of the bend at the center circle. Record the angle.

4) Bend the first angle. With the new piece of tubing in the tubing bender and parallel with the straight edge and the middle of the  bender over the center hole, bend to the needed angle.

Using a protractor and tubing bender to duplicate the needed bends


5) Make the second bend. Put the old tubing against the new and determine where the inside radius of the next bend will be. Now make a longitudinal line on the new tubing . Get the measurements of the distance from center bend to center of bend as well as the angle of the bend. Now place the tubing , line down, in the bender and again bend it to those specs. Repeat the procedure of bend 2 for bend three. Now take it out and check the bends for fit.

Looks like a good fit


     Now put a flare on the end of the replacement tubing as done on the vehicle end of the line.

Fitting and finishing the install

1) Install the new piece of tubing on the brake hose.

2) If necessary bend the car end brake line back to the mating position with the new line.

3) Install the double flare union on the old brake line. Don't forget to use some anti-seize lubricant.

The finished double flare union


4) Turn the brake hose end of the line into the other end of the union.

5) Tighten all fittings to final tightness.

6) Bleed the brakes. A four wheel brake bleed is necessary since the reservoir was drained.

7) Test for leaks. 

Here is a link to an inexpensive flaring tool kit with good reviews:
                                                                                                      basic bender
                                                                        






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