Air dryer

The air drier, especially for installing in compressed air systems of commercial motor vehicles has a drying cartridge (14) installed in a housing (12), and a filter unit (18) separating a pre-filter section (22) from a post-filter section (24). A check valve (26) prevents an air flow from the pre-filter section into the post-filter section by bypassing the filter unit, and which allows an air flow from the post-filter section into the pre-filter section by bypassing the filter unit with proportionate pressure ratios so that a transporting of foreign particles (28) from the post-filter section into the pre-filter section is made possible.