The newer steering knuckles/hubs have a double seal. But guess what? They fail too with vacuum leaks after a period of time.
Best solution? Continue to rebuild the front spindles with new bearings if needed. But don't install the vacuum hubs. Purchase Warn manual hubs, part number 60247, and install them instead. Then disconnect the vacuum soleniod electrical connector and tape it off. Remove the vaccum lines in the fenderwell. Plug the vacuum port on the back of the hub with a screw and epoxy. You can leave the steel vacuum line projecting into the fender open.