So the SS-5Form no help with a location.
Do you have their Canadian naturalization? Any luck locating the arrival manifest to Canada? What about the church your father and four of his siblings were baptised in? I would think that the church baptismal record would provide you with a parents place of birth. http://www.kdejsme.cz/prijmeni/Posp%C3%AD%C5%A1il/hustota/
look in the left column at the names as how popular/common they are. http://www.kdejsme.cz/
click on the name.
I'm sure the Praha of old had more than one church just like now. The church records are indexed by a location and then religion.http://actapublica.eu/