Oil or brake fluid on the shoes will make then lock up easily. If there are no leaks and the car has always done this, then yes, a proportioning valve in the rear line will likely cure the problem. It is better to do that then to back off the adjustment.
To balance brake the brake system on the old cars it was done with wheel cylinder diameters. If the wheel cylinders have been changed out over the years and the wrong ones installed on your car it can cause the problem you are having.