What is the secret to getting new brake lines at the master cylinder and distribution block leak free. Lines and distribution block/proportion valves are new from classic industries. Lines are steel.

Tighten the heck out of them to get the flares to flatten out and seal
Make sure and use a tubing wrench only on them, not an standard box or open end wrench
Good luck, let us know what you find please.

I like to tighten them up and then back them off slightly and repeat that process several times, getting them tighter with each turn.

I know this sounds wrong. But trust me it works. Over 25 years, no leaks. A dab of white Teflon sealer on the flare and thread. I don't know why it works. My feeling is it lubricates and allows the parts to seat. I didn't believe it when I was told years ago by a friend. Got frustrated with leaks one day and tried it. Never looked back since.
Doug

i have had good luck using hi-temperature copper anti-seize on the threads and using a toothpick to apply just a tiny dab to the flare.

Get a flare lapping tool.

Wow! For the toolbox that has everything!

I have a different, Parker brand, tool for that that I forgot about

At $150 I'll just tighten and loosen a few times to seat the flare.

https://www.speedwaymotors.com/Koul-Tools-P-51-Sure-Seat-Lapping-Tool-Kit-37-45-Degree-Flare,246739.html

Wiser people shop around:

https://www.summitracing.com/search/part-type/flare-lapping-tools

Not what you posted. Try telling us how much the one you posted costs, hint $150, I shopped around.

And have used this procedure on MANY lines over the last 30+ years. Do not over tighten to the point of strippin the hex or threads. Good and tight but not brutally so. What you will observe if watching carefully, is that each time you tighten, loosen, and retighten with the same torque the Nut will move further than before. as it is forming the "SEAL" between the opposing flares. After repeating several times the nut will stop moving as much.
The finishing tool John showed will likely help but I've never needed one.
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Another tip if the leak continues, carefully look at the male and female flares. You'll likely see the "SEAL" problem normally on the softer of the two.

Thanks for all the advise. I’m try a “poor man” lapping devise I made using valve lapping compound and steel wool. Hope it works.