I'm currently working on all the variations of this name as well.
I know that my great grandfather came to the US from Austria in 1905 as Michael Hajduk. His wife Charlote it seems followed in 1907 (though, can't find her ellis island records yet)... they went to live in Taylor, PA.
My grandfather (John) .. and his siblings (Charles, Mary, Steve, Peter (Pat), all at one point, and on the 1920 census went by Hydock.
Some family members believe it was also spelled Haydock, & or Haydak ... though I haven't found anything supporting this as of yet.
At some point most of them changed it to Hydeck, though Pat went by Haddock as well. Not sure why he changed it yet again.
I'd love any information or hints & clues that pertain to this.. and hope this helps someone else as well.