To comment a bit further, the gas ballast is a leak valve that admits air (the drier the better)to the pump. In 2-stage pumps the leak is between the stages. It doesn't really expel vapor as such. When moisture laden air comes into the pump, during the compression of the gas in the pump, the vapor (if the concentration is high enough) will liquify, contaminating the oil, etc. The leaked air dilutes the water vapor laden air and inhibits liquification.
Obviously some water will always liquify, the ballast just lengthens the time between oil changes. Nothing's perfect.
If you are a purest, it's best to vent your system with dry air or nitrogen to keep water out, period. This is great in industrial applications for repetitive processes but amateurs are always taking their stuff to pieces so dry air venting doesn't do a thing. But, things will work better in New England in the winter than Florida in the summer.
Steve
Created on Wednesday, February 28, 2001 11:17 PM EDT by Steve Hansen