The best way to inflate the sanders is with King Arthur’s Tools Guinevere small hand pump, which allows you to feel the softness of the bulb while you are pumping, giving you the ideal drum softness for the grit being used. The best pressure is as soft as possible while still keeping the sleeve from moving on the drum.
Guinevere inflatable drums require very little air, so it’s very easy to over-inflate them with a compressor. The compressed air system must be able to deliver totally dry air, otherwise moisture will cause rust on the internal steel parts, and will interfere with the airtight seal of the interior valve.
The sander actually becomes flat when pressed against a flat surface and round when pressed against a round surface i.e. the sanders conform to the underlying surface. Consequently the sanders should be filled with very little air. The #120 Drum has a 3/4" Diameter and is 1" long with a 1/4" Diameter Shaft.