If you can feel/hear/sense a rear brake release then it's almost certainly rear e-brake shoes finally unsticking and relaxing into place, not a frozen or sticky caliper piston. Could be a badly worn drum and shoe getting wedged into place with built up grunge, but much more likely the entire cable and adjusting/release mechanism is near frozen with grunge and a bit of rust. For starters, remove the rear rotors and clean everything up. If you remove the shoes for cleaning or replacement then it can take a bit of prying to get everything back into place. Lots of WD-40 or whatever until the mechanisms and cable ends all move freely then clean it all up with spray brake cleaner. Pulling the rear rotors with a badly grunged mechanism and worn drum brake areas can take a bit of effort. Beating on the drums with a hammer and cleaning everything up from the back side as best you can may be where you need start in order to get the shoes pulled back enough to pull the rotor off. In a really bad case a puller maybe needed. Once everything is cleaned up and smoothly releasing then you can do a proper e-brake cable adjustment at the hand brake.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now