Brake System light revisited

The piston does self center when the pressure is the equal on both sides...which it will be if the brake system is not experiencing any problems.

As for the drawing that shows the added valve as a 'proportioning valve', that is interesting as B Bodies have the same valve and it's called a 'hold-off valve'.
A hold-off valve allows the rear drum brakes to apply first before the front discs begin to apply. A prop valve reduces rear drum application in proportion to front disc application, the goal being to reduce or eliminate rear wheel lockup.