It’s not impossible, I’ve done similar brazing jobs when I’ve had no options.
I might repeat myself but here are my suggestions
make sure all the mating surfaces are completely back to bare metal (no corrosion, plating, paint etc)
that they are clean of dust and any other deposits (a drop of oil will cause failure of the joint)
Ensure the surfaces mate uniformly to one another (I.e. that there are no burrs on edges or depressions in surfaces)
Ensure a good fit, the surfaces must be snug but with sufficient movement to generate the capillary action required for brazing to occur. With tubing this is loose enough to allow the pipes to slide by hand but tight enough not to wobble.
Most of the work is in the preparation. The temptation is to rush to the heating stage however, those that do rarely have success. A little more time and effort with cleaning and ensuring the correct fit will save a lot of frustration later.
Make sure you have the correct filler rod for your job and the right flux.
Construct a method of containing and concentrating the heat around the parts. Fire bricks are best but failing that use some form of fired masonry. I’ve used terracotta tiles, old red bricks, clay pavers and on one occasion, a large broken terracotta pot. Don’t use stones or anything made of concrete, it can explode and injure you badly.
Get the steel to a dull orange (beyond red heat but not bright orange) and then apply the rod to the steel and allow it to melt and flow into the joint.
Don’t try using the torch to melt the rod, that will just make a mess and even if some joining does occur it will fail under any load.
Best of luck!
