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.
