Have you checked the operation of the slider pins on that caliper? they can bind, especially if the rubber boot is stuffed and when you put new pads in, it used a new part of the shaft which could have dried. Easy enough to fix, pull bolt off, remove boot and clean/lube with grease.
I'm hazarding a guess here and assuming the rea brake setup is the same as EA's. The handbrake setup is pretty dodgy on these.
Check that the leaver on the back of the caliper is resting on its 'stop' on the caliper (its a square raised section cast into the caliper body), if its not the piston is being pushed causing the brake to engage slightly.
Did you screw the piston in as far is it would go?, it will auto adjust itself after.