[boygenius]

Are your rubber rear brake lines new or in good condition. If they are old they might be collapsed inside. The high pressure fluid will go one way to the caliper but will have a hard time flowing back to the master cylinder. When you look at the pistons in th ecaliper do they look shiny or are they a dull grey color. They should be smooth and shiny and free from any pitting. If they are very dirty or pitted the pistons won't return and the pads will drag. You could also still have some air in the system and as you drive the car the air expands in the caliper and pushes the pads out.