Abstract: A peritoneal dialysis system is provided that includes a hardware unit, a disposable unit received by the hardware unit, and a controller. The hardware unit includes a recessed area, a piston having a contact surface, a pneumatic source for supplying a negative pressure, and an actuator configured to move the contact surface of the piston into and out of at least a portion of the recessed area. The disposable unit includes an outer member fitted at least partially within the recessed area of the hardware unit and a moveable membrane positioned between the contact surface of the piston and the outer member when the disposable unit is received by the hardware unit. The controller is configured to cause the pneumatic source to apply the negative pressure to the moveable membrane so as to conform the moveable membrane to a shape of the contact surface of the piston and follow the contact surface when the piston is moved towards and away from the outer member fitted at least partially within the recessed area.