Don't toss them out yet, lots of us have drums on the rear that perform perfectly. You just have some problem and it could be in the master cylinder, bleeding, lines, residual valve, etc.

Start at the source, the master cylinder and make sure it is truly putting out enough flow from that port. Maybe you have the ports reversed, or the wrong master cylinder. I bet if you spend all that money for discs the problem won't go away. Just make sure your whole system is matched and working properly.

Don