I always replaced toro stuff. have you contacted Toro Direct? I can on monday while at work if you would like. had a return toro no flow control valve returned few weeks ago for non operation. actually 2 come to think about it.
I’ve seen this cracking too. But Haven’t noted though any instances that the cracks are causing the valve to not operate. We usually have a valve weep, then we will switch out the top end all internal guts. I’m pretty certain this piece just keeps the diaphragm seated properly and not becoming deformed.