Brake problem on 99 gmc sierra 4x4!?

I have a 99 gmc Sierra pickup and there was a rusted out brake line on the rear drivers side. So I replaced the lune and went to bleed the brakes but there is no pressure what so ever coming to the back. The front works fine but the back will not get any fluid to the line. Could it be the master cylinder or something? I doubt that there is any blockage in the lines.