You might try jacking the front of the frame as high as you can then bleed the rears. Try to get the master cylinder as high above the drums as you can. This may help get the air out. Also, tighten the rear shoes up tight against the drums just so you know any movement in the pedal is air and not brake shoe travel.
Rustednut has a good point, where is the proportioning valve? I installed a adjustable one at the rear on my truck so I could limit the pressure to the drums. My would lock up the rear brakes to easy. Its easy to adjust with just a twist of a knob. A few test stops dialed it right in. Joe
