I think your right about the heat. Like unterhausen pointed out, there is bare steel where the brass pulled away from the HT. The tubes were hot enough to wet out but in hindsight, not consistently. The oxy/mapp works but you spend lots of time getting things hot, then you have to change oxygen cylinders, then start again. It worked for a lugged frame that I've been riding for over a year but I think I'll get a better set of oxy/acetylene before trying to fillet braze again.