The root cause may well be dry nylon bushings (#4 in the drawing) on the pivot shaft. Pulling the pedal and lubricating the sleeves should resolve the problem. You might also want to connect a medium tension return spring as indicated number 11 in the attached line drawing from the 1960 shop manual:
Mine was originally a GMC V6 and came stock with this spring, there was no over center spring as shown in the detail drawing in the upper right corner. Without the return spring mine will not consistently return to full pedal height.
