After installing a 12 bolt Chevy rear and a 4 speed Saginaw transmission, I still had the same brake problem. So I decided to take another look at the new master cylinder. This paid off for I finally discovered why my brake drums were locking up! There is a washer type part on the end of the MC, which the brake pushrod goes through and which is held in place by a clip spring. Well, I discovered that this washer was backwards. This meant that it took up extra space within the MC and caused the primary piston cup to be moved towards the center of the MC. In this position the primary cup now blocked the compensating port in the MC. This meant that when the MC is activated, brake fluid trying to return to the MC reservoir, would now be prevented from doing so. Just turning this washer around cured my brake drag problem.
After I purchased the new MC, I took it apart in order to paint the case with POR 15. I guess I put the washer in backwards when I reassembled the unit. This error has caused me much grief, so I hope that this post will help prevent others from making the same mistake.
