Have a 96 silverado, 2wd 1/2 ton 350. Front brakes started dragging one day to the point of doing a roadside release and using e-brake to get it home. Replaced the front calipers and pads and started to bleed them when ran into problem. Front bleed great but rear is getting no fluid or air. Replaced the rear lines and bleeders but still nothing.
Check your brake hoses in front, they may be colapsing and leaving pressure on front brakes. Then power bleed system it maybe an abs valve thing. Dont' be surprised if you need 2-3 qts of fluid to do this.