The real key is the fluxing procedure, the stainless requires a hydrochloric acid type flux, and the copper needs to be clean, and pre tinned prior to the braze attempt, then the parts are assembled, and the steel part heated, and the joint fed with the braze/solder. This keeps the oxide layer down on the stainless surface until it reaches the wetting/bonding temp of the solder.
these splain it somewhat, but the last link is the kicker....
and of course, it comes from BEER BREWER's....who are next to gods in the heirarchy of life....
found this in my searches;
"You can use EUTECSIL 1020XFC, which is elastic flux-coated silver alloy for low heat joining of ferrous and non-ferrous metals ..
You have to heat broadly along the joint line with a carburizing oxy-acetylene flame, then touch 1020XFC rod to joint until the flux is melted off and then feed it into joint, keeping the flame cone roughly one inch away, and continue melting the rod until a continuous fillet is observed ..
Allow deposits to solidify, then quench in water and wash away flux residues .."
and also this;
"Yes, you can solder stainless steel and copper together. It's done all the time in breweries. It takes a special flux. Use a lead free solder. First tin the copper with solder. Apply flux to the stainless steel, and fit the two piece togethers. Then heat the joint, while feeding more solder into the joint once it is hot. Use a propane torch. It's important to tin the copper first, then flux the SS. Using this method will protect the SS from the air until it is hot enough to apply the solder. Don't over heat the SS since it will oxidize and become brittle."
here's the good stuff:
http://www.brewingtechniques.com/library/backissues/issue2.6/palmer.html