On brazing jobs that fail - 99.99% of the time, it's cuz the welder failed to braze properly.
One does NOT melt the brass with the flame ... EVER.
One uses the heat from the base metal to melt the brass. It is the mistake that is most commonly, and easily, made. Gotta git it dull cherry red, every single time. *s*
Brazing is soldering - same principle applies with lead solder joints. Ya git a 'cold solder' joint which fails, electrically insulates or doesn't work - every single time one uses the soldering iron to melt the solder, rather than heating the junction, and letting the base metal melt the solder.
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