Tommy is right... You can easily TIG copper to steel, especially outside corner joints. 347, 321, 308, 309, 316, even Hastelloy W (wonder rod) fillers all will do fine. Fillets are very difficult and grooves only slightly less difficult.
But the original question mentioned "Brazing" so I was thinking about lap joints which would be much more difficult for GTAW...
However, I have used GTAW for joining stainless to copper with various brazing fillers without flux (both high and low temp), but always for argon backups and welding fixtures, and art :) Never for any project that had an engineered load or a failure would cause problems.