between opposing surfaces 34 36 of the stator assembly and carrier respectively to control gas leakage between the arm of the stator assembly and the carrier surface 40 of the carrier mounts lands 42 44 of the labyrinth type back up seal 14 the lands may be comprised of an abradable structure such as honeycomb corresponding knife edges 52 54 of the labyrinth type back up seal are carried by the rotor assembly with respect to the rotor assembly rotor assembly 18 supports the seal runner 26 which is annular in shape specifically the rotor assembly includes an arm 56 that extends from a mounting bracket 58 mounting bracket 58 facilitates attachment removal and or position adjustment of the rotor assembly the knife edges 52 54 of the labyrinth type back up seal are supported by an annular extension 60 that extends from the arm of the rotor assembly thus extension 60 assists in defining an intermediate pressure cavity 62 that is located between the hydrostatic seal and the back up seal note also that extension 60 can assist in preventing debris e g debris that may by attributable to unintended damage of the hydrostatic seal from passing beyond the back up seal in a normal mode of operation i e when the hydrostatic face seal is properly seated the desired pressure.