1981 300CD Brake Dragging still w/ new caliper

Aloha guys,

Back brake started dragging intermitantly for about a month, replaced both back calipers. Now it's worse! I can't even make it a mile before it's egg fry hot. Had to pry the caliper off, could not get the piston to return until I pulled the brake line off, then it went in smoothly.
Is there a valve or obstruciton preventing fluid return? It's only the back passenger side. I'm ready to disable the back brakes if I can't figure this out!

Thanks
Chris