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@s.walrus
Mr Jangle, It looks like your flux is spent, it has either been over cooked, or just heated for too long. Try using more, and remember to add more, if necessary, during the brazing.
The blackening might be the result of a slightly carbonising flame, so try a little more oxygen in the mix.
Some brasses work better for capillary work, and some for fillets, so make sure you've got one that is free flowing. I used SIF Bronze 101 for capillary, and SIF Bronze 1 for fillets, they behave very differently, 101 melts lower and flows well. Also use small diameter rod for capillary as the whole rod melts evenly easier. I used the Cycle Designs LFB flux and found it lasted quite well if I didn't heat pass a nice vivid orange. If you've not already seen it, this video is quite good:
[ame="http://www.youtube.com/watch?v=u94WsKf6JRo"]silver
brazing lugged bicycle joint with MAP gas - YouTube[/ame]
The video shows silver with MAPP, but everything he demonstrates worked for me with brass and oxy-propylene mix. I think with oxy-acetylene, with it being even hotter, you need to keep the flame moving more(that is don't forget to move the flame around, to prevent over heating, while you are feeding in the filler).
The one place that made actual, real MAPP stopped in 2008! Now it is all propylene mixes called MAP or MAP-Pro or such. They can work fine for brass if used with oxygen with the right torch mixer.