Abstract: A compressed air supply system includes a compressor and an air dryer connected to receive compressed air from the compressor. The dryer includes a desiccant bed through which the compressed air flows to provide dry compressed air, and a blow-through valve operable to connect the desiccant bed to atmosphere. Secondary and primary reservoirs located remotely from the dryer are connected to receive dry compressed air from the dryer. Control components integral with the dryer are provided for controlling a charging operation by controlling air flow from the compressor through the dryer for charging the primary and secondary reservoirs with dry compressed air. The control components also control a purging operation by controlling air flow from the secondary reservoir through the dryer desiccant bed and through the blow-through valve to atmosphere in order to purge the dryer, while maintaining an initial air pressure within the primary reservoir.