What is the part called that all the brake lines run into before going to master cylinder on 1988 dodge omni?

That part is indeed called a proportioning valve. It has a valve inside with a spring; it adjusts how much braking pressure is applied to the front and rear brakes. In the case of an Omni, the rear end is significantly lighter than the front and the rear brakes have a tendency to lock up before the front. This valve helps prevent that. They are no longer available from the dealership and as far as I know nobody in the aftermarket makes them. If you need one, wrecking yard/used is your best bet.