If it is brass, You may have problemes due to the zink content, if it is bronze there will be less or no zink, and it will be easier. Tig or oxy fuel torch and silver solder or brazing are Your best options if You need full strength, silver solders usually work at a little lower temp than brazing rods. If strength is not much of an issue You might try soft solder and a torch.

Another option, depending on the type of repair is epoxy, if the service temp at the repair is low enough.

Yes, there is such thing as cast brass. For pump material, however, bronze is more often used than brass.
If a yellow fume develops when you start welding, it means that the material is brass. Yellow fumes are made of zink oxyde.
Of course, you can file a few decigrams of the metal and run a chemical analysis. A chemical lab will take just a few minutes to confirm what material is it.
Giovanni S. Crisi
Sao Paulo - Brazil

I have tiged some brass pipe fittings such as the ones on the end of a fire hose before. I did it with a ranger 250 scratch start. It worked pretty well but I started with the heat way low and worked my way up. Brass has a low melting point I guess.