You need to solder it, not braze. Use Silvabrite 100 or any other silver bearing solder. I would also suggest a silver bearing tinning flux such as Oatey's. The most important part is not overheating the stainless and causing the surface to rust. It's like soldering to a copper plated Holdrite bracket but even more finicky. Heat your coupling first and you'll be fine.

To be honest, if you're uncomfortable with this you may want to just rent a propress tool or use a compression fitting. I've soldered to the stuff in crawlspaces and basement ceilings, not sure I'd be okay burying it in a wall. You can always tin the end of the pipe first and then put your coupling on.

For a brief period in time copper was more expensive than stainless due to the housing boom. Look up "TuboTron" Stainless Steel pipe.

 

Lord help you if you run into Tin coated steel "Bundy Tubing"!

 

I would be ok with using a compression fitting as a back up. I think I will go try a couple joints in the garage to get an idea of what I ned to do.

 

I’d use the compression fitting. That said, I’ve never worked on radiators so I’m not really qualified to have much of an opinion. I try to stay in my lane.

 

That all looks like copper to me in the photo.

 

That was my thought as well. It can be difficult to tell until you sand the old stainless pipe as it will develop a patina and look real similar to old copper. But I still don't see what could be stainless in his pic. I was just taking him at his word that there's soldered stainless he has to deal with.

 

I don't have much experience working on radiators, so I prefer to defer to someone more knowledgeable in this area. I believe in staying within my own expertise.

 

I have actually ran into another issue. I filled and purged the system, I turned up the stat for heat to make sure there were no other problems.

Now the boiler won't kick on when there is a call for heat.
The stat clicks when its turned up.
The circulator is running.
Damper is open.

My 3 guesses are:
Thermocouple
Flame roll out switch
Blocked vent switch

The thermocouple I will test to see if its producing the milivolts

For the FRS&BVS, can I do a continuity test or test for 24 volts?

Its a 1 zone house so only 1 stat. The pilot is a standing pilot that is on.

Is there anything else I should check? Boiler is Burnham.

Thanks.