I have seen two instances lately where someone was trying to use silver flux for brazing with brass (LFB). Here's why it doesn't work.
The working temperature of LFB is between 1670°F and 1750°F (910°C - 955°C)

The working temperature of Nickel Silver is 1720°F - 1800°F (938°C - 982°C)

The working temperature of gasflux type b is 1400 ºF – 2200 ºF (760 ºC – 1205 ºC)

My experience with Type B with nickel silver isn’t good though. Not sure what I’m doing wrong. OTOH, the late, lamented Cycle Design nickel silver flux works great

Moving on to silver fluxes,

The working temperature of gasflux type g is 1050 ºF – 1600 ºF ( 565 ºC – 870 ºC)

The working temperature of Harris black Stay Silv is 1050 - 1700 deg F

The working temperature of Harris white Stay Silv is 1050 - 1600 deg F

So the only one that has a temperature range overlapping with the temperature range needed to braze LFB is black Stay Silv. You have a 30F degree range though. Good luck with that. If I was doing this every day, I might be able to make it work half the time. The first time I bought lfb from my lws, they tried to get me to buy Stay Silv. That’s the only brazing flux they have. I wonder how many people they have discouraged by selling them that with LFB.

Selling silver flux with brass ("bronze") is just incompetent. I don't expect retail clerks to be highly trained, but my LWS is petty and unfriendly too.

I like giving a brick&mortar shop my business when I can, so they're still there when you need 'em, but I'm done with my LWS (Central Welding in Seattle). They have disappointed me one too many times.

I went in to buy wire for my MIG, and they only had 2 lb spools. Huh? A shop with Welding in their name and they're out of welding wire? Never heard of a 2 lb spool before, but I thought, at least this'll get me through the day, so I bought it. Declined the receipt because I usually do most places, I don't need the paper.

Got it home and opened it, turns out 2 pounders have a smaller hole thru the middle, and there's no way to put it on my welder(*). Confirmed with an email to the maker of the machine. Disappointed that the LWS clerk didn't tell me that, and I did tell him what welder I have. No prob, I ordered an 11 pounder on the web and it came in a few days.

(*) yes I found out there are adapters to put 2 lb spools on a larger MIG machine, but I hear they don't work great. Maker of my welder advised against using one. Gimmicky, inconsistent drag makes for feed problems, and they cost more than I paid for the 2 pounder. I'm generally against throwing good money after bad. You can also re-spool wire from a small to a large spool, but I value my time more than what that 2 pounder cost me.

I brought the 2 pounder back to the LWS, and they refused to give me a refund! Even in store credit. I explained to the guy that I only bought it a couple days prior but "no receipt, no refund". Unfriendly! Do they think I stole it and I'm trying to finance my meth habit or something? I was going to buy a pound of TIG wire, but no way will I ever give them another dollar. Back to the web shops for that too. I told him I'd been shopping there for years but I'd never come back if they stiffed me, and he said "fine". Left the useless 2 lb spool with him, I don't want it taking up space in my shop for the next 10 years until I think of a use for it, it would just be an ugly reminder every time I looked at it.

I'm so glad I switched to propane and an O2 concentrator, so I never have to buy gases from them either.

I did that once! I got the filler to flow, too! (tho not well.) I was just making a thingum to hold a braze-on, one of those "maybe I'll use this again someday but likely single-use" ad-hoc tools. I hadn't brazed in a while so I forgot which flux was which. I picked up a yogurt tub, that flux had been decanted into because the original jar it came in cracked. The flux looked gray so I though it was for brass, turned out to be just dirty silver flux.

I figured out early on that it was the wrong flux, but sort of brute-forced it. The joint is a bit ugly but plenty strong enough for the application.

I'm pretty sure brass doesn't need a lot of flux unless the part is exceptionally dirty/rusty. I did some experimenting on a throw-away project with only a little flux and no cleanup, and it flowed out fine and displaced the cruft. Type b is pretty good for that.

The guy that posted his brass brazing that he did with silver flux on facebook got a lot of comments about how he overheated the joint, blah, blah, blah. Technically he did, he overheated the flux. And technically correct is the best kind of correct.

My LWS has gotten better over the years, it was mostly that one bad experience. They even replaced an acetylene tank for me because it was leaking, I don't know how. They said it couldn't leak, but bring it back.

Never thought about an oxygen concentrator, but it sounds like a great idea. Can you enlighten me on the specifics of your unit?

I'm not Mark but I've been promoting the use of an oxygen concentrator for many years. They are more convenient, more safe and more cost effective. I like the Devilbiss 525 model or its predecessor model 515. Some Invacare models work well too. The Devilbiss models have a greater Oxygen output. While prices vary a typical lower Craigslist price is $200 although if you are lucky they can be found for much less. You can buy refurbished ones for more. Because they are often estate sales the relatives that inherit a concentrator are open to bargaining. In the medical industry they are really expensive so sometimes a seller will base their price on that rather than for non-medical hobby use. They are common in the "lampmaking" hobby. I think that is about melting/blowing glass. This subject has been covered extensively on all framebuilding online forums. A few quick searches will provide you with a ton of information.

There are 2 cons to consider, the 1st is that they have to run a few minutes to purge the line of regular air. I turn mine on when I start to flux. And their output is not enough to run a big rosebud. They have enough oomph for a standard size rosebud. Because they are such a sensible choice, I've taught all my framebuilding class students on using them.

Yeah everything I know about them, I learned from Doug.

Rather than re-typing everything you need to know, I hope it's OK to send you off to a different forum, namely the framebuilder subforum on Velocipede. There's a years-long (I think) thread there that might take a long time to read, but you'll be well-versed by the end.

I now have two of them. (I had 3 but sold one!) One is a 5 liter per minute (l/m) unit and the other 10 l/m. The 10 l/m units are far less common; you'll find a hundred fives before you stumble on a ten on Craigslist. The reason I keep both is to run two torches, one of them a rosebud for quickly heating up something heavy. Then when I'm at brazing temperature, I hang the rosebud on a gasaver, a switch that turns the torch off when you hang it up. This is an extreme luxury for a home-gamer. No one needs that! But I got spoiled by having that setup, one place I worked. I got almost all my stuff dirt-cheap (mostly CL), but even if free, two O2 concentrators take up a lot of space in the shop, so believe me I am not recommending that.

BTW, to OP or anyone else reading this, if you're near Denver, I might have a lead on some reasonably-priced used ones. A guy on another forum (the framebuilders googlegroup) mentioned that he bought one from a guy that had a couple dozen for sale. Not rebuilt, just used. He charged $250 for a 5 l/m Devilbiss, but he also said he'd stand behind it, and replace it with another one if it didn't work. Lemme know if interested and I'll put you in contact with the guy who posted that.

How expensive is silver solder nowadays? My first couple years in college I got a job working in a jewelry manufactory making silver jewelry. I found it worked very well on steel, with much cleaner results.

Do you really mean solder? The stuff we're talking about is brazing filler, not solder. Though the word solder does get used for it sometimes, mostly by laymen. Even some framebuilders have been known to make that mistake, so don't feel bad if brazing is really what you meant.

Probably different in other countries, languages, maybe even in different industries within one country. The French cognate is souder, which can mean solder, braze or weld depending on context.

As to "much cleaner results", that can go either way, depending on things like joint prep and the skill of the brazer. A brass braze can be equally clean, often cleaner, since silver flux has less cleaning power than brass flux. Though a skilled operator can make perfectly clean joints with either.

I only know this because I recently took a jewelry craft class as a local maker space. Jewelry uses different filler with generally higher silver content (57%-75%) than we typically use for framebuilding (<56%). The pricing seems to be in the same ballpark . The jewelry folks sell it as 5 feet of 26 gauge wire rather than 1 ounce of 56% filler like we do for framebuilding. YMMV.