i have a 1977 f600 dump truck and have had the same issue before. i just kept bleeding the brakes after i was sure they were adjusted correctly. i would bleed them until there was no air. then id drive the truck around my property working the brakes. about 20 min trips. after returning id bleed them again and find air. after about three times i had 1/2 pedal the first pump. hope this helps. does yours have 6'' rear pads and dual wheel cylinders? if so do you know the part number for the lower wheel cylinder. i have been unable to find one for my truck. thanks