A medical backcheck valve includes a hollow housing containing an upwardly biased piston assembly for controlling liquid flow therethrough. The piston assembly comprises a substantially rigid plug having one end protruding from the upper end of the housing, and a flexible sheath covering having a relatively small diameter upper end surrounding the plug, a main seal for controlling flow through said first flow space, and a tubular lower end, secured at the bottom of the housing, for preventing liquid from entering within the covering.