The others have already said it, but I think the 2 main things you can do are:
1) Use pre-flared lines from e.g. NAPA or Autozone. These come in standard lengths that you can couple together.
2) Buy or borrow a set of brake line wrenches. You'll need this anyway when you get to bleeding your brakes or else you'll run the risk of stripping the bleeder nuts.
Making a good flare is just another one of the skills you'll need to build a car. I'm all about learning new skills. Once you have the tools and the skills, you'll find other uses for them.
Double flares have to be nearly perfect before they'll seal. Start by getting a straight and clean cut on the tubing. I like to use a tubing cutter. But some people say not to use them because they harden the ends.
Once it's cut, clean the ends to remove burrs and junk.
Set the tubing into the jig carefully, and make sure it's not too shallow or too deep. Do the same thing for the second half of the flare. If you crank the handle down too tight, you'll crack the tubing, and then it will leak for sure.
Practice on some spare tubing until you get it right every time.
Oh, and buy some tubing wrenches. They're cheap at Sears, and worth the money you spend on them.
Just a different bit of advice as everyone above had good suggestions...
Remember, it's just a car... don't let it beat you. Take one line/leak at a time and work it. Whether that means replacing fittings, the line, or cutting and reflaring, keep at it without letting it get the upper hand.
I had a few leaks on my brakes and had to replace one line (the one with a bubble flare at the master). We've all been there and done that, so we know how it feels.
In the end, building a car is like eating an elephant... it's a big job, so take it one bite at a time.
Just to chime in. As others have stated, get a set of brake line flare tools to prevent damage to the flare nuts. these look like a box end wrench with a slot in them to allow you to pass the brake line through the wrench end.
If the flare looks good, then tighten the flare nut with a flare wrench a lot tighter than you think they should be. A flare won't seal unless they are TIGHT. You almost can't tighten them too much.
Nobody has asked or mentioned anything about what type of flaring tool you are using. You really need a quality tool to start with or your chances of making a good flare will be greatly diminished.
I do agree with what has already been said though, use preflared where ever possible. Even though I have alot of experience flaring tubing I always use pre done unless there's no way around it.
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