Hy WVPharmer, "(also impossible to go backwards)", could you have your brake shoes on backwards, should be the shoe with the shorter lining on the forward side of the backing plate on all brakes, please check that that is the way they are. Now adjust the booster to pedal rod to have a little clearance to make sure that the master cylinder can return to its correct at rest position and uncover the compensating port, adjust the brakes and try them again. As others have suggested you may want to remove your master cylinder and bench bleed it just to be sure there is no air trapped in it, if you do do that then you'll have to bleed your brakes one more time. Once you get your brakes working correctly you may find you are getting more rear wheel lockup than you like, you can install a proportioning valve in the line to the rear brakes to stop the lockup. Most manufacturers have installed an adjustable proportioning valve in their pickups rear brake line to give the vehicle less rear brake when unloaded, hope that helps.
