A shuttle valve mechanism for a twin tower, desiccant containing compressed air cleaning and drying system housed within a rectangular cover member attachable over desiccant containing chambers to form a closure thereover with a slide valve chamber therein containing a reciprocal valve member with passageways interconnecting the two ends of the slide valve chamber with the two desiccant containing chambers. The reciprocal valve member is provided with an aperture to permit a purge percentage of the air dried in one chamber to be admitted into the other chamber to effect a purge cycle whereby the purge percentage will remove moisture from the desiccant.