Abstract: A pressure diffuser is constructed so that the compression of pulp that normally occurs during rapid movement of the screen assembly does not significantly degrade the efficiency of the washing (or other treatment liquid) action in the diffuser. This is accomplished by providing at least two headers, including a first vertical header and flow control assembly for one or more sets of horizontal treatment liquid conduits adjacent the pulp inlet to the diffuser, and a second vertical header and flow control assembly for one or more horizontal treatment liquid conduits remote from the pulp inlet. The first and second vertical headers may comprise a common pipe with a flow preventer precluding flow between them, or distinct pipes supplied with treatment liquid by a common pump. The advantageous results are further achieved by providing at least one ring [mounted between the outer screen and the concentric inner support cylinder of the screen assembly] with a plurality of rapid-flow restricting openings.