Since brass (or more appropriately) bronze-alloy rod requires the base metal to be heated red before it will bond, unless there's a really lean cylinder or two causing the manifold to glow, braze repairs stay put very well. Brass is also less likely to create stress risers at the edge of the repair and cause a crack to form beside the weld bead like nickel rod does if it's not properly applied. Brass flexes during the heat cycling that an exhaust manifold experiences during warmup and cool down every time the engine is started. The main concern is getting the base metal really clean to allow the brass to get a good bond to the casting. Some poor-quality cast iron has so many impurities in it that it's impossible to either weld or braze- - - - -just scrap it and find another part!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!