The pressure diffuser incorporates a screen that has a considerable weight that is moved by a connecting rod in a reciprocating manner between an upper and lower position. The connecting rod is maneuvered by a hydraulic cylinder for vertically moving the screen in a first slow stroke concurrent with pulp-flow and moving the screen in a second fast stroke countercurrent to pulp-flow. The pressure diffuser has an inlet defined therein at the bottom of the vessel but the hydraulic cylinder is located at the top of the vessel. This design enables an increased active pressure area in the hydraulic cylinder for the fast stroke of the screen. This reduces the power-rating needed for the hydraulic system.