http://hebrewbooks.org/pdfpager.aspx?req=14678&st=&pgnum=21&hilite=
I don’t know where the story comes from, but it is said that the Gra, who was a b’chor, used to be podeh himself from every Kohein he met, until after he met a Kohein named Rappaport whose yichus he trusted and he stopped being podeh himself after that.