Brazing aluminium is easily 10x more difficult than steel. That's why small torche kit doesn't include alloy rod; anyone doing it is going to pro-welder with decades experience and has big pro torche.
You have to sand both sides to bare metal to remove the oxide layer. Wipe with acetone to remove all grease and oils (use gloves to avoid skin oils from getting on it). Flux used should be matched to base alloy, not generic like steel brazing flux.
Then temperature cannot be determined by glow of metal since aluminium doesn't glow when it gets hot. Very narrow window between proper temp for brazing vs. couple more degrees and you've melted it into useless puddles. While claims of alloy brazing as having strength in 45-60kpsi range, in my experience, it's closer to 30kpsi, or just about 1/2 of 6000-series alloy. Not really worth it, unless it's non-structural decorative item.
Welding is really only way to join aluminium reliably with strength. While you can TIG weld alloy with reverse-polarity DC (cheaper machine), it's A LOT easier with high-frequency AC unit. I can actual weld beer cans together with one!
Starting out, I suggest starting with brazing some steel items together 1st. Much, much easier to learn that way. Flux-coated brass rods removes some of difficulty with mixing and applying proper amount to joint so you can focus just on heat and rod.
I'll post series of photos on how to fit that foot-peg into rearsets.