I bought a brake flaring tool from harbor Freight and when ever I flare the end of a brake line I end up cracking it. Is there something I need to do to prevent this that maybe I am not doing or maybe I am doing it wrong.

Basically what I do is put the line in the clamp put the bit in the clamp and tighten so it forms. everytime I have done this it cracks the line.

Might be a flaw in the tool ... but where did you get the brake line? In order to form the proper flare, you need to start with annealed (soft) Bundy tubing.

Usually the stuff sold with fittings already installed is too hard for successful flaring by hand.

Also don't try to make the flare too big (which is set by how far the tubing protrudes from the clamp). I forget the proper distance offhand, but it's not much.

I have the Harbor Freight tool and it works OK, but you must set the protruding bit of the pipe to the same height as the bit except for the small diameter stem that fits inside the pipe.
That's not easy to explain but I hope you get it. My tool had an explanatory diagram with it .

A couple of hints. First after you cut the tube use the pointed reamer tool that's usually part of the cut off tool to clean up the inside of the tube end and remove any rough edges. That will spread the stress from the flareing die more evenly around the tube. Also run a file across the end and the outside edge to smooth off any rough surfaces that could cause a stress crack to start. Then dip the end of the tube in some clean brake fluid before placing it in the flaring tool clamp. That acts as sort of a lubricant and won't contaminate the final system. For a simple flare the tube end should be flush with the face of the flaring tool clamp, for a double flare the end of the tube should be above the surface of the clamp the same amount as the thickness of the double flaring die for that size tube. To make a passable British type bubble flare it's the same height but use the flat side of the clamp instead of the chamfered side.

I bought a winding of line that was made to be cut and flared. I think that a couple of things were the problem, i had the line sticking out too much and it wasnt clean enough. Thank you very much for the replies I will give it a try tonight and hopefully it works.

Also thanks Bill I didnt know the there was a difference for using sides of the clamp.

This makes one less tool I bought that I dont know how to use!!!

Bill's recommendations are spot on, clean the end to be flared well, file it so it's flat perpendicular to the tube. I use "Swiss" files (small, different shapes) to clean the I.D. and #220 wet-or-dry to smooth it. Cracking is the result of stress risers, cleaning and proper prep makes a BIG difference.

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Practice, practice, practice!!