Master cylinder replacement snafu - help

[CW4B]

Decided to replace 1/4” brake lines between MC & distro block. Couldn’t find prefab line with correct fittings for one of the connections, between the rear MC fitting (nearest vacuum booster) and distro block. So I searched around and found a “dual MC adapter” that, after everything connected and brake pressed, the “adapter” leaked like a siv(sp).
See pic. Sorry, puc didn’t work.
Evidently wrong adapter but that was the only one that would even screw into MC that also accepted the tub fitting.
Any suggestions?
Adapter = AGS BLF-25C 9/16-18
Prefab brake line = 7/16-24 to 1/2-20