How do you get the brakes to work on the rear of 1993 GMC truck the master cylinder booster wheel cylinder proportioning valve have all been replaced and still very little fluid pressure?

there is not ment to be alot of pressure for the rear brakes, because they are self energizing once they are applied. the brake shoes in the drums are set up to the primary shoe starts to rub on the drum and then it pushes on the other one so it is engaged.