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.
Whenever sanding sleeves become clogged, use the cleaning stick to increase the life of each sleeve. Regular cleaning can extend sleeve life by between 10 to 15 times.
Turn the motor on and position the cleaning stick at the top of the sanding sleeve. Use any corner of the cleaning stick to press against the sleeve then at a medium pace and light pressure, pull the cleaning stick back down the sleeve. This motion rapidly removes wood dust.