Brake fluid leak

#1
One of the steel brake line feeding the rear brakes is leaking a bit of brake fluid.
The leak is located at the base of the firewall where all the brake and fuel lines enters the engine.
Brakes are still operative but I have to fill up the resevoir when the brake warning light appears.The brake lines appears to be rust free except for this very spot where the rustproofing company did not put any.
Anyone replaced a steel line on these cars before? What is the size of the steel line, is it a double flared connection? Can I make my own?
 
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