Hi all.
Back onto more familiar ground from that of the Mercedes Benz chocolate fireguard!
Using a silver solder to braze steel presents no problem. The common difficulty is not realising that steel needs more heat to reach brazing temperature. A little patience or a bigger burner and a longer life flux eg HT5 normally overcomes.
The difficulty with leaded steel is that the lead is not alloyed but is present as little globules of free lead. It sits there on the surfaces of the joint. As the joint is heated, the lead melts. When the silver solder melts later, the two mix.
Lead in silver solder makes it less fluid or free flowing. It can affect the penetration of the alloy into the joint and hence the joint strength.
Reduce the effect of the lead. Dilute the lead content in the filler metal by having a larger joint gap say 0.2mm.
Thereafter carry on as normal. The same principles for successful brazing apply. It's still all about capillary flow.
Keith