My paternal grandfather was Juan Simon Diaz born 16 May 1881 in Gómez Palacio, Durango, passed away 16 January 1974 in Oakland, CA. I never met him, but his son Salvador A. Diaz (1915-2012) was my father. My father always said that Juan Simon told him that HIS father (my dads paternal grandfather) was Porfiro Diaz's brother. Did Porfirio have a brother named Simon Diaz? (My dad may have been incorrect about his grandfather's name, but not the relation). We beleive that's who Juan Simon was named after.
I've seen pictures of Porfirio and his eye region looks very similar to my late father's.
Further information: Juan's wife (my paternal grandmother) was Paulina Francis Loera (1893-1932). They all came to the US in 1923 via Texas.
As a side note, Juan Simon reportedly had a sister who died young. She became a nun and while at her convent, the story is that she sat on a knitting needle. The wound became infected and she died. Does this story sound at all familiar to anyone? I've never met any of my father's family, so I have no way of getting additional information.