Bike pump: what is this lump?
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."