I am not sure which rear brakes you have BUT, on 55.2-59 2 Ton, you need to un-adjust the upper shoe on the rear brakes before you bleed. The top shoe on both sides needs to be all the way in (loosened away from the drum so it pushes the piston all the way into the wheel cylinder, to get all the air out).
http://chevy.oldcarmanualproject.com/shop/1955truck2/55ctsm0507.htm
