[Mark McLeod]

The "in" for the KOP compressor is a felt patch on the side. There's no place to attach any fitting, and the draw there isn't all that great anyway.
It's not one of the two ports opposite each other on the top of the compressor. Those are both output ports.

A Venturi works because a flow of air is constricted by the device to speed it up. An increase in air velocity is matched by a corresponding decrease in pressure. The faster moving air draws a vacuum. Like blowing across the end of a soda straw.

So continuously forcing compressed air through the Venturi device results in a vacuum. It doesn't take a great deal of air to make it work.