If you have problems only with the rear it could be a bad proportioning valve that is only working one way. If you have no joy here then check for pinched or crushed brake lines . It could also be a bad master cylinder that will not allow revers flow of brake fluid .However this would cause both rear brakes to size at the same time. It will also be a good idea to change the brake fluid if you've heated the brakes beyond there normal operating temp . Once its overheated it can begin caramelizing . My thought on this though is that you have a bad proportioning valve. Hope this helps