There is a flat spring inside the caliper that is suppose to retract the piston a fraction of an inch when the pedal is released. It sounds like that is what went bad causing the pad to be partially engaged at all times.

 

Under normal operation, the caliper piston is retracted by the piston seal. The seal is a rectangular cross-section that pitches towards the rotor at the caliper piston surface when the caliper is pressurized. Once the pressure is released, the seal will return to a rectangular cross-section pulling the caliper piston back slightly.

This youtube video illustrates the above point:

I prefaced that by stating "under normal operation" as corrosion will definitely impact the retraction of the piston. Corrosion of the caliper/piston can cause excessive drag on the piston to piston seal and/or the piston to caliper bores which will overcome the force the seal can exert to retract the piston.

Reman calipers can have another issue, that being an oversized bore which can allow the piston to twist in the bore and get stuck. Some remans were simply cleaned up and assembled with new seals/piston(s) while others required a light hone to clean up corrosion which may have taken away too much material. The rear calipers on these trucks are finicky things.

This is all assuming that it is the piston getting stuck in the bore. The caliper brackets can often be the point of failure in the rear brakes on these things. With the cost of the caliper brackets being less than $20 each, I would recommend replacement to anyone that has corroded brackets. If the bores get gummed up on the brackets, the pins cannot freely float the caliper which is required for proper release of the pads. If the outer pad is wearing away prematurely, it is pins that are at fault for not allowing the caliper to float after the piston has been retracted.

On a few occasions I have used emery cloth taped to a drill bit (after degreasing) to clean the bores in the brackets prior to regreasing the pins/bores. To ensure that the pins work to full depth without binding, they should be checked without their boots installed and rotated all the way until they bottom out. The key here is to use the minimal amount of grease.

Another thing that has caused grief on other applications (not s-series, but worth mentioning) has been the isolator ring that is on one of the caliper pins. This ring can get excessive grease behind it and bind to the pin bore in the bracket. I always try to install these rings without any lubricant to limit this occurrence.

If the pins are moving freely, it sounds like the caliper simply needs to be replaced with a new or another quality reman.