I believe the PO could have a brake hose problem. Usually a hose will separate and cause the caliper to hang up from pressure not being released.
One thing I have seen many times is a brake fluid bottle setting on a shelf for a long time and then used to top off a master cylinder. Brake fluid attracts moisture when left on the shelf. Never buy more than you are using and throw the remaining in the trash. You will contaminate a clean system with old fluid.
4 piston Kelsey Hayes calipers totally release and allow the rotor to spin free. Single piston calipers have a constant drag. Also the pistons do not ride on the bore they ride on the seal installed in the bore. They are pretty easy to rebuild but are available at AutoZone for $129 per side. Usually the caliper housing where the pad rides is wore and the pad gets hung up on the housing where it is suppose to slide. Under the gold tension clip pictured below.
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