...My estimate is that Packard sold almost 150,000 cars with Treadlevac as it was a popular option. It was also installed on many other makes including Continental and Mercedes.

They work great with a very short pedal travel in normal driving. In an engine-fail emergency, they are no different than any other vacuum assisted power unit. You have to press hard. The only weak spot in the design is that they will not tolerate dirty lumpy brake fluid.