Doug, where is this located? If it is in the compartment below the driver seat, it may control the step slide, or the air ride seat. I know you said you checked all items. I don't think I would plug anything, because the an air leak to the environment usually indicates a failure of the module, diaphram ect. Try disconnecting the input line to the module (pressure side), then re-check to see what does not operate. OF COURSE, DO NOT DRIVE THE BUS WITH THIS DISCONNECTED UNTIL YOU KNOW WHAT IT CONTROLS!