Legendaries can drop at any level (possibly excluding very early level, I know some guns like elemental ones don't seem to drop in single digits). Note that farming is very slow for some guns as the chances involved are single digit percents, possibly lower than 1% in some cases.
I know a friend was farming Bunker for quote some time and I dont' believe he ever got what he wanted. I have personally found legendaries not very worth grinding for (because they take a long time not because the guns aren't good). Instead I just beat up raid bosses or bunker/warrior because they give a good amount of Purple guns and sometimes, if you're lucky, a legendary, but usually not the specific one you want. Playing with friends seems to help the odds a fair bit though.
If you're farming Bunker remmeber to just save and quit without leaving the Bunker area to make him respawn, don't leave the area or you have to make the long trip again. Save and quit and you should spawn at the Bunker map.