I should mention that Gasflux Type B is available in a powder form. It is very useful when you need to apply flux in the middle of a braze. You heat up your rod hot enough so when you dip it into the powder it will stick - but not so hot the rod will melt. It can also be a bit tricky to melt it off the brass and onto the joint. Again you have to apply enough heat to the flux on the rod to melt it off but not so much as to melt the rod itself. Usually that requires a rotating as well as a back and forth motion.
Many years ago before I heard of the Gasflux company, I used to buy brass flux from my local welding supply store. It came in a powder form in a can that had a separate small hole in the middle of the lid (so it had like 2 lids). This was for sticking your hot brass in as well as just a general holder for shorter stubs. I still have mine I place near me when I am brass brazing in case I need to add a bit more during my braze. I never brass braze without it being close by.