I know this isn't what you want to hear, but the way you're describing it, it really sounds like adjustment and air in the system.
It could also be the design differential front/rear pressure ratio of the proportioning valve. My '87 was built with JB5 brakes. Braking was so poor that you would pray you were going to stop in time with the pedal nearly mashed through the floorboard! Very scary! I'm surprised there was never a recall. The brakes stayed that way until I reconfigured them. Now, it stops on a dime! Many GM trucks see three or four-to-one wear ratio, front-to-rear.
Even so, consider some of these basics:
* Your brakes could be out of adjustment, or less likely not correctly arced to the drums making for poor contact.
* Your rear drums could be worn past their useful service life (effectively, making the rear brakes out of adjustment).
* You may still have air in the system. - Try bleeding the lines
at the master cylinder, then bleed them again at the rear wheels.
* The master cylinder could be bypassing internally.
- When you 'lightly' step on the brake pedal and hold it, is the pedal solid or does it gradually fall toward the floor?
* The proportioning valve internal piston may be tripped or sticking.
- Does the brake warning lamp work in the dash when you set the parkbrake with the key switched ON?
- With the Ign ON and parkbrake
released, does the brake warning lamp light up when you step hard on the brake pedal?
* You could have a leaking brake line leading to the rear axle.
- Have you had to add fluid? How do the lines look?
* The rubber brake hose between the body and back axle could be damaged internally (a flapper).
I expect some others will chime in with some good ideas.
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