If you type "leaky flare fittings" into Google, you'll get nearly a quarter million hits. Why? Because flares leak. A lot. Many of those Google hits will be discussion threads from enthusiasts wondering why all the flares they spent a weekend creating all seep. No one can tell them WHY, but I can.
My car sat for 4 years while I tried to figure out why all the flare lines to my prop valve kept seeping. I tightened and re-tightened. Diassembled, fiddled and re-tightened again. My reward for all that fiddling was worse seeping. I didn't need the car, so I parked it and ignored the problem. Then it happened on another Honda I own. Finally this last week I replaced the prop valve and re-flared all my lines. The result? MORE LEAKING!!! One fitting just wouldn't stop.
Here's the fitting in question:
The theory behind flares is that you fit one metal cone inside another metal cone, then smash them together hard enough that they deform together and seal. If that doesn't sound like it would work very well, you're right. It doesn't.
If you read around on the web, you'll run across articles (like this: Brake Line Flares: Metric & SAE, Inverted & Bubble - Race & Track Driving (formerly Win HPDE)) that delve deep into flare angles and whatnot, trying to explain how it all works. If you look at what really happens when you squish two flare cones together, however, you'll find that all of this is irrelevant. The reality is that the leading edge of your hard line will smash a microscopically thin, flat ridge into the cone of your flare seat. That's the seal, and it's a wonder it works at all.
Here's a good cutaway diagram of a flare seat. You can see the nicely shaped cone.
Here's what that nice seat will look like after you've tightened your fittings one time.
You can see the crunchy ridge that has been crimped into that nice cone. Angle means nothing. The only thing that seals is that tiny thin crimp. Cones, angles and whatnot are just a lot of blahblah hocus pocus. Double-flares are nothing more than overly complicated compression fittings. Loosen that crimp one time and you'll be lucky to get it to seal again. If, like most enthusiasts, you swap brake parts out for different components you got from the junkyard, you're going to have problems. The flare seats on the junkyard parts are already deformed to the brake lines on that junkyard car, not yours. My own problems started when I replaced my LX-i prop valve with an SE-i prop valve I got from the junkyard. When I looked at the flare seats in the new prop valve, they were mashed completely flat. There was no sealing power left there at all.
You can also see that the sealing ridge is not centered on the cone. That is because a number of different variables have to come together in just the right way to seal a flare fitting. For one, the flares produced by your cheap Chinese flaring tool have to be centered on the brake line. Under ideal circumstances, that's not likely, but if you're working against the firewall down at the bottom of your engine bay, it's almost impossible. Second, your brake line must be perfectly perpendicular to your seat BEFORE you tighten the flare nut. The flare nut will NOT pull it square. Given that your line has been bent every which way already, and two other lines are already pulling the tee out of alignment, it's highly unlikely you'll get it right.
If things don't come together just right, you'll have a seat that looks like this:
Notice that there is a gap in the sealing ridge. That gap is my leak. The only solution is to over-torque the flare nut until you create a sealing ridge. Honda's torque spec for their flare nuts is 11 ft-lbs, which isn't much. Sadly, because Honda used such crappy flare nuts, you'll round them off before you get much beyond 11 ft-lbs. I replaced Honda's crappy pot metal flare nuts with stainless steel, so I was able to over-torque my flares to 30-40 ft-lbs and create a seal.
To sum up, beware when you're collecting brake parts from the junkyard. You're likely to have problems with them. It's much better to buy new parts. If you're already having problems, you can try over-torquing. Just remember that Honda's crappy flare nuts will probably round over before they seal. If over-torquing is not an option and you're wondering what to do, inspect your connections. The hard lines are steel. Unless they are split or broken, you probably don't need to replace them. The flare seats are brass, by contrast. They are what deforms, so if you're dealing with seeping lines, replacing the seats (ie: the component into which the hard lines connect) will probably fix your problem. In other words, you don't need to replace your hard lines for seeping.
Long story short, double flares suck. Which is why all the hot-rodders use AN fittings instead. They are a single flare connection, which I've found to a be a LOT more forgiving than double flares (disclaimer: I have not worked with AN brake lines, I have worked with single flare A/C lines). If I have another big brake job, I will probably swap everything over to AN and throw all the double-flares in the trash where they belong.