A rotary drum for thickening pulp from a slurry, the drum including drainage channels delivering filtrate from a pulp mat on an outer surface of the rotary drum to at least one filtrate chamber at an end of the rotary drum, a flat face valve and plenum chamber assembly in the filtrate chamber. The flat face valve juxtaposed against drainage outlets of the drainage channels, wherein the flat face valve does not block the drainage outlets during a majority of the rotation of the rotary drum. At least one opening on the flat face valve extending through the front surface and aligned with the drainage outlets to provide a fluid passage through the flat face valve and to a plenum chamber behind the flat face valve such that air in the filtrate is collected in the upper portion of the plenum chamber and removed from the rotary drum.