Duster Rear Wheel Cylinder Size

Could be the multi function valves proportioning valve is bad?

Go to rock auto and search for your cars rear wheel cyl. In the "info" link many have the bore size.

Also rear drums will most likely lock up before the front disks due to weight shift and the self energizing effect of drum brakes.

It sucks but that has been my experience with all my rear drum cars from 63 Econoline van to 2007 Taurus