The ABS unit consists of the computer (Kelsey Hayes), the control valve (brake pressure modulating valve or something like that) which is underneath the computer and pulses the brake pressure and the combination valve on the side that the brake lines from the master cylinder thread into. It also has the pressure switch on it that activates when the front or rear brake circuits experience a pressure loss.
Earlier versions (1994 or maybe 95 and earlier?) had a difference setup.
You'd need to replace/duplicate the combination valve in the new setup.
They sell them individually, a lot of older car owners install them when going from front drum to disc. Search for a proportioning or modulating valve or something like that.
I forget the correct names of the valves, but with a disc/drum setup, there has to be a delay in either the front or rear brake activation since disc and drum response times are different. I believe the drums have to be allowed to engage first else the discs would do all of the work.
The valve allows that to happen, so you wouldn't just pull the entire ABS unit and plumb it to the master cylinder without installing a new valve.
