Interesting drum brake problem

A somewhat far-fetched but plausible scenario:

Drum brake systems use a residual pressure valve in the line. When these cars were built, the RPV was in the outlet of the master cylinder. Later on, Mopar changed to putting the RPV in the rear wheel cylinders. It is at least possible that the cylinder on one rear wheel was changed to the later type out of ignorance. I don't know for sure what the effect would be, but a failure to release pressure seems a possibility.