My trusted old Renault\Opel\Vauxhall\Nissan (lol) Vivaro\Trafic\Primastar brakes locking up at the rear and cooking brakes & wheels, time to do something. No doubt it was the return mechanism for the pistons after use of parking brake.
Water\salt\dust\rust must have found it's way deep into the piston push+retract system for the park brake (rubber seals getting hard?). The normal brakes where fine though after their 17 years on the car, zero rust on the pistons.
2x brand new shining calipers off ebay only 95 euros! & surprise with removal of old parts, no stuck bolts etc to battle with for half a day! Bleeding with a cheap kit with non-return valve at the end of the tube. 1 h work for the first, finding the tools etc, 1\2 h for nr 2.
So, what a great day! things for once working out much better than anticipated :O)