The pin could be a release for the metering valve.
That's very different than a proportioning valve. A metering valve is used to hold off the initial application of the brakes to the front under light pressure. This helps reduce lock up on icey road - more so on certain vehicles than others.
More about the different components in here: 1970 Imperial & Chrysler Brake Hydraulics Facts Reference Book (Session 274)
Chrysler valves for '72 are pictured on the last page of this booklet. http://www.imperialclub.com/Repair/Lit/Master/293/page13.htm
