As mentioned, although called a ‘proportioning valve’ those units are actually more of a distribution block, used to keep the front and rear hydraulic brake systems separate when a dual master cylinder is used. They also house a valve that when it senses a large pressure difference between the front brake hydraulic pressure and the rear brake hydraulic pressure it lights a warning light. That Summit part looks pricy, I bought the same one from In Line Tube probably 5 years ago and works fine, no leaks. I don’t remember it being expensive either. You may want to look there.
