1994 Van Diagram/Schematic for braking system

I have a 94' Van its like a G20 but the shorter version. It has ABS Front disk and rear drum brakes. The problem is when you depress the brake pedal the rear drums lock up then they let go and the van keeps going forward as I am depressing the brake pedal. The brakes are very sensitive. None of the idiot lights are on for the ABS so am I right to assume its a mechanical failure from the drums? I would appreciate it if anybody could give me a diagram for the braking system on this model van. Any info or tips on where to look or what it might be please post. So far I have replaced the master cylinder and put new front brake pads. So the fronts good.