The metal brake line beetween the caliper and the rubber line seals using a flare fitting. The threads in the caliper are not what holds the fluid back. The short steel line either needs to be replaced of the brass seat in the caliper is bad.

It may just be coincidence that after the tape was applied the leak stopped or the tape let you get the fitting a bit tighter and it sealed at the flare.

Even if you get it to seal at the threads it would leak where the tube goes through the nut. A brake system is no place for teflon tape.

Kevin