Replace the brass with zinc.
Jeffery, brass is copper plus zinc, unless you're talking about a
leaded free machining brass (which certainly would make for brittle
gold). If you simply replace the brass with zinc, you're reducing
copper and increasing zinc. That won't improve problems with
brittleness, as the zinc doesn't particularly reduce brittleness.
it's there as a deoxidizer, reducing oxides and porosity, but more
than the small amount needed to do that just makes the gold alloy
paler in color, lower melting, and not quite as ductile/malleable.
And because of the dramatic differences in melting point between gold
alloys and zinc, the easiest way to add zinc as a deoxidizer to gold
alloys is to add it in the form of a copper/zinc alloy, which of
course is brass. (I'm repeating myself, I know... )
So I'd guess the problem is not with brass itself, but perhaps with
the TYPE of brass, or with some other contaminating factor.
Now, if the brass in question is actually a bronze, copper and tin,
then getting rid of that would make the metal dramatically better,
since tin is not so good for gold alloys...