What is this lump in the picture and how does it work?
I have another pump, which physically is the same but minus the bulge. The one in the picture seems to perform better, ie less work to reach the same pressure.
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This from an old article circa 1996 it references a Medai floor pump. "Screwed into the base near the compression tube is an air tight steel chamber. This chamber is known as a reservoir and holds a volume of the compressed air to assist in equalizing the pressure of the pumps compression chamber and the inner tube. This feature gives the tire a small assist in time as it "seats" on the rim."