I put an AirTex or whatever the name is, double unit on my truck several years ago. It leaked gas around the housing rim no matter what I did. I got a replacement and it leaked gas and oil. Both were junk.
The vacuum unit only enhances the vacuum when the engine vacuum is really low, like at a wide throttle setting. The rest of the time the engine is drawing more vacuum through the pump. Also the pump is only useful at higher RPMs. If I remember correctly the pump by itself draws about 11 inches.
