Brake shoes dragging passengers side rear

This is my Grandson"s vehicle so I only see it when he has an issue. His passenger side rear brake stay partialy applied and when he arrived the pads were wore down to the metal. I replaced the pads, rotor, and caliper after removing all rust and build up that may have interfered with the free movement of any new contacted parts. Everything moved freely as it should have, Fast forward 700 miles. He arrives again with the same issue. The brake components were so hot the caliper piston boot disintegrated. I took all parts back and had them replaced under warranty thinking the piston in the caliper stuck and installed the new parts. Fast forward 1000 miles.I checked the wear on the brake pads (passenger side) and found they were 1/2 gone, the pads were replaced on the drivers side at the same time and were not noticeably worn. I checked the rotation with the tire on and noticed a slight drag then took the tire with rim off and there was still drag turning the rotor. The caliper moved freely on the mounting pins with the pads and rotor moving with it.I then took all parts off and looked for anything that may have obstructed free movement or brake fluid flow and found nothing.I then checked to see if the fluid flow from the drive side rear brake at the bleeder valve was similar to the passenger side and it seemed similar-it was maybe a little better than passenger side.I tried to trace the brake line from the passenger rear brake to the reservoir but lost track entering the engine area. I did not see any defects in the line.I took the reservoir cap off and applied 110 pounds of air pressure to the brake line in an attempt to clear anything from the line. The reservoir fluid did not rise. I AM BAFFLED? Any suggestions would be appreciated. Thank You