Catalysis of the reaction: (8R,9Z,12Z)-8-hydroperoxyoctadeca-9,12-dienoate = (7S,8S,9Z,12Z)-7,8-dihydroxyoctadeca-9,12-dienoate.
(8R,9Z,12Z)-8-hydroperoxy-9,12-octadecadienoate mutase ((7S,8S,9Z,12Z)-5,8-dihydroxy-9,12-octadecadienoate-forming) activity