The part often called a proportioning valve is actually a Pressure Control Valve. It is part #12 above. It functions as a T and provides fluid to the left and right front brake calipers. It is completely independent of the rear brakes. It has a piston, a spring, and seals inside. These valves are now fifty years old and most likely frozen. Wilkinson explained to me that they tend to trap air and result in a spongy brake pedal. He sells a brass T that eliminates it.