leaking Brake lines
Someone mentioned fittings type. THIS IS IMPORTANT
You must be certain you are not mixing SAE flare and AN / JIC flare fittings.
SAE is 45 degree flare angle
AN / JIC is 37
Might help to lube the fittings before assembly. Research this, I think you need to use mineral oil, as some types of oil are not to be used with brake systems.