Great thread!
I don't know if this is too technical for the interwebs, but what sort of flame you're using for fillet brazing? I've had relative success with a soft flame and the sequence melt brass puddle, back away and let solidify, add on top, repeat going around... But my fillets are kinda small. Too strong a flame and the fillets melt away on the side, but even with the soft flame I can't stay long enough to build a large fillet...
when starting out learing to fillet a softer slower flame is easier to work with, it slows down everything to give you a chance to pull back before your puddle gets away from you.
as you get better a faster more neutral flame lets you get it done fast, which is important to maintain the strength of the tubes. also the neutral flame allows you to to have a bigger puddle which goes to the root of the joint, and will give you a fillet that requires less finishing.