A source of compressed air, such as one or more air compressors and/or one or more tanks of compressed air, is used to sequentially fill a plurality of compressed air storage cylinders. A plurality of pressure-sensitive, one way check valves cause the storage cylinders to be automatically filled up, one at a time, in a determined sequence. The outputs of the storage cylinders are connected through a plurality of differential air pressure valves to a fill station, to cause the storage cylinders to fill up individual air breathing tanks, in which the storage cylinders are depleted in a determined sequence, one at a time. By depleting first the first to be filled storage cylinder, first in—first out, the same filling procedure can be used to refill the air storage cylinders.