A medical treatment system, such as peritoneal dialysis system, may include control and other features to enhance patient comfort and ease of use. A fluid trap of a peritoneal dialysis cycler may have a valved connection to a vacuum source or a positive pressure source. The fluid trap can be connected to the vacuum source in order to sealingly engage a flexible membrane of a pumping cassette to a control surface of the cycler during a therapy. Any fluid entering a space between the membrane and control surface can be drawn away and collected in the fluid trap. The fluid trap alternatively can be connected to a positive pressure source in order to aid in separating the membrane from the control surface to facilitate removal of the pumping cassette from the cycler. The fluid trap may include a liquid detection sensor.