A peritoneal dialysis system includes: a hardware unit including a piston having a contact surface, a pneumatic source for supplying a negative pressure, a stepper motor, a controller including a processor and memory programmed to control the pneumatic source and the stepper motor, and a video monitor operating with a touch screen for sending and receiving information to and from the controller; and a disposable unit including a moveable membrane positioned adjacent to the contact surface, the programmed processor and memory operable to (i) cause the pneumatic source to apply the negative pressure to the membrane so that the negative pressure moves the membrane to conform to a shape of the contact surface and (ii) control the stepper motor so that the membrane follows the piston as the piston is moved away from the disposable unit and moves with the piston as the piston is moved into the disposable unit.