Compressed air supply system

A compressed air supply system for a heavy motor vehicle, said system comprising:

• a compressor for supplying compressed air;
• an air dryer connected to receive compressed air from said air compressor, said air dryer comprising 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;
• a secondary reservoir connected to receive dry compressed air from said air dryer;
• a primary reservoir connected to receive dry compressed air from said air dryer;
• control components for controlling a charging operation by controlling air flow from said air compressor through said air dryer for charging said primary reservoir and said secondary reservoir with dry compressed air;
• said control components also controlling a purging operation by controlling air flow from said secondary reservoir through said air dryer desiccant bed and through the blow-through valve to atmosphere in order to purge said air dryer;
• a one-way check valve connected between said air dryer and said primary reservoir in order to prevent a loss of pressure within said primary reservoir during the purging operation;
• and
• a pressure protection valve connected between said air dryer and said secondary reservoir in order to prevent a loss of pressure within said secondary reservoir during the charging operation, and to allow for a loss of pressure within said secondary reservoir during the purging operation.