Did you grind a V groove along the crack, and take all the surface rust down to bright metal for at least 1/2" or so on both sides? Rusty cast iron is almost impossible to weld (or braze). Getting a good bond on the initial pass is essential. Once that is done, plain old 6011 all purpose steel rod can actually be used to build up secondary passes. Weld a 1/2" long bead, peen for 5 minutes for stress relief, preheat- - - -weld- - - -peen- - - -"rinse and repeat". Then bury the part in a pile of sand overnight for a slow cool-down.
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!