The proportioning valve determines how much fluid is pushed into the brake lines. A slight brake pedal application pushes a small amount of fluid toward the wheel cylinders/calipers resulting in light braking. A heavy brake pedal application moves the proportioning valve further in the master cylinder which results in a larger amount of fluid being pushed toward the wheel cylinders/calipers. Due to the increased resistance to the outward movement of the shoes/pads in these heavier braking applications, higher pressure builds in the caliper/cylinder and brake lines.

Guess that all I can say aboout this subject. Anybody else have any input/corrections/comments? Always open to learn new ideas.