An air dryer unit includes an air storage tank including an elongated hollow body of a one-piece construction having a partition wall disposed therewithin to divide the interior of the tank body into a first chamber and a second chamber in an air-tight manner. The second chamber has an outlet connectable to a pneumatic circuit. An opening is formed through that portion of the tank body defining the first chamber. An air dryer device includes a dryer including a tubular housing containing a desiccant and extending through the opening into the first chamber, and a hollow cover member detachably coupled to the dryer housing to cover that portion of the dryer housing extending outwardly through the opening. The cover member is detachably coupled to outer surface of the tank body and has an inlet connectable to an air compressor. One end of the dryer housing disposed in the first chamber communicates with the first chamber while the other end communicates with the cover member. Compressed air from the air compressor is introduced into the inlet and passed through the cover member and the dryer housing into the first chamber. Means is provided for communicating the first chamber with the second chamber for flowing the compressed air from the first chamber to the second chamber. A check valve is associated with the communicating means to prevent a backflow of the compressed air from the second chamber to the first chamber.