a method for operating a dialysis cassette including a flexible membrane that covers a pump chamber includes applying a vacuum about a piston head and sucking the flexible membrane of the dialysis cassette onto the piston head ; mechanically retracting the piston head along with the vacuum applied membrane away from the dialysis cassette to draw fluid into the pump chamber ; mechanically extending the piston head and the flexible membrane into the pump chamber to expel fluid from the pump chamber through a flow path ; and directly sensing a pressure of the fluid flowing through the flow path at a location of the dialysis cassette adjacent to the pump chamber .