The Bendix® AD-9 air brake dryer is a desiccant type (moisture absorbing) air brake dryer. The dryer consists of a die cast aluminum end cover secured to a cylindrical steel outer shell with eight cap screws and nuts. The end cover contains a discharge check valve assembly, safety valve, a purge valve housing assembly, which includes the heater and the thermostat assembly. The air brake dryer has three female pipe thread air connections. The ports and connections are identified as follows:

In the charge cycle, the compressed air flows into the air brake dryer through the supply port (SUP/11) where baffles change the direction of the air flow several times. This constant changing of air flow reduces the temperature and allows the contaminants to condense and drop to the bottom of the air brake dryer end cover. The air is then directed through a desiccant cartridge, where water, oil, and other contaminants first flow through an oil separator which removes the liquid water as well as the oil, the oil vapor, and the solid contaminates. The air then exits the oil separator and flows through the column of desiccant. The air becomes progressively dryer. The majority of the dry air exits through the internal check valve into the delivery port (DEL/2) and out to the supply reservoir.

In the purge cycle, the air compressor unloader port is connected to the air brake dryer control port (CON) which opens the purge valve in the bottom of the air brake dryer. This action allows the water and contaminants to exit through the exhaust port in the bottom. Contaminants are blown out by air stored in the air brake dryer housing during the charge cycle. The complete purge cycle takes about 15 - 30 seconds.