I have never seen one of these working, but during the 40s chevy offered a vacuum assist on the clutch. By the time I started working on cars they had all broken and been replaced with standard linkage. I have seen some pieces and it appears to have been a vacuum bellows in a cannister that pulled on the clutch linkage when the driver depressed the pedal. My first car, 48 chev 2dr, had one at one time but it was gone when I bought it in '59.
