The last thing I repaired (by brazing a crack closed) was a Conn 20K "gooseneck" (upper mouthpipe) with a c. 1-1/2" long crack (actually a tear) in it (roughly halfway up the tube - that ran semi-diagonally roughly in the direction of the tube).
After running dent balls through it, straightening it out, and getting the two sides of the crack nearly closed, I held the bracing together with bailing wire (re: very high heat) and successfully spanned the long crack with silver brazing wire. It wasn't "pretty", but (as the shape inside and the base metal surface was restored to the original shape), I COULD HAVE filed and sanded it down to "pretty"...
...The band director asked me to replace it...I did, but (after sorta halfway sanding/buffing the repair) I offered him the original one back as a spare, at which point he asked me if he could just pay me to repair that one, and not buy the new replacement. I shrugged my shoulders, told him that I'm trying to get better at repairing cracks without lead-soldering brass patches over them, and told him to just take it - "my compliments" (and put the new one back into the Conn sousaphone necks drawer).
I've done other crack brazing repairs - lately - whereby the only thing that could be noticed was the color difference at the point of the repair...ie. I took those to their cosmetic conclusion.
The Conn sousaphone gooseneck was an over-the-top (probably ridiculous) repair, but I just wanted to see if I could repair a crack that long and that bad.
Just as with any soldering or brazing, the main thing is to control the heat. In the case of spanning these cracks, JUST BARELY ENOUGH HEAT to get all involved surfaces "happy" with each other is the key. It's getting easier. Large flatter surfaces are more difficult (such as the front curves of bottom bows, or bell flares. Hopefully, I'll get a bit better at doing those. I can do them, but I would like for them to end up being closer to invisible... Again: I can float that silver-bearing solder over such places, but would rather perfect actually brazing, as it's so much more strong and durable.
oh well...Thanks for reading the blather.
I guess (??) I'm trying to learn to really master this for the same sort of reason that I abandoned "C" tuba and am working to master the B-flat tuba...It's "something to do" to keep (otherwise, getting-to-be pretty mundane) stuff interesting.