A RPV is designed to maintain a slight amount of pressure in the brake lines at all times. On rear drum brakes with strong return springs that retract the shoes when the brake pedal is released, and relatively small diameter wheel cylinder cups, a 10 PSI valve is needed to keep the wheel cylinder cups expanded against the walls of the cylinder to avoid air getting past the cups and a spongy pedal developing. Since the only thing that tries to retract a disc brake caliper piston is the distortion of the square O ring as the piston pushes against the pad, and the large diameter of the piston, a 2 pound valve is all that can be used without causing the disc brake to drag. Most of the hydraulic brake light switches I've seen were either located at the master cylinder outlet fitting on single-line brake systems, or in the rear brake line like 1950's MOPAR vehicles. Virtually all auto manufacturers went to pedal-mounted brake switches in the late 1950's or early 60's, mostly because of the proximity to the steering column, and the necessity to incorporate a discriminator circuit to the brake lights on vehicles factory equipped with turn signals.
Jerry
