Hi - I have a distant connection with Iowa Hrabak's. I'm going to list some standard sources that may help narrow in on your grandfather.
1. The 1930 Census on Ancestry is searchable on names. There is an Estella M. Hrabak born in 1901, living in Belle Plaine IA, with a son born in 1925, but the son's name is indeciperable, but middle initial B.
2. If you know your GF's death date, his death certificate should give his birth date and place, and might give parents names.
3. Check the SSDI. Ancestry has a version, as do other online genealogy sites. If found, then you could request a Social security application, which should have parent's names.
Your GF's name may not have been Bruce originally, as people may have Anglicized an old-country name.
I'll also send you privately, the names of some people who have large family databases with some Hrabaks in them.