I really struggled with this yesterday. I came to the conclusion as some point that the metal is not "yellow" brass. I was able to add LFB on the end of the bar with no problem. The conditions seemed to improve as I added more LFB. The thing I didn't pick up on is the LFB was pure and clean looking, the bar not so much.

I machined the bar to the correct size and polished the surfaces. The color matched and there was only a little porosity. I fit everything up nice and clean, fluxed, added heat when the flux was ready the brass wouldn't take the silver (commercial 45%).

I went up and down with the heat a little and things didn't improve. The flux when cooled turned a solid grey colored with balls of solid black. The metal, whatever it is has been all over the place and was originally purchased from an online source. I think it is type 360 and not weldable.