A medical valve assembly includes a tube extending between a first and second tube end along an axis, and a plunger plate extends radially from the second tube end. A valve housing surrounds the tube and includes a radially inwardly extending flange. A compression member is biased against the plunger plate and compresses an elastomeric seal from a non-compressed condition to a compressed condition to establish a sealed condition of the medical valve assembly. An inner surface of the elastomeric seal in the non-compressed condition has a plurality of planar portions and a plurality of radiused portions, with adjacent planar portions interconnected with one of the radiused portions to improve a closure of the elastomeric seal during compression. The inner surface preferably includes three planar portions and three radiused portions to define a generally triangular-shaped inner surface as viewed in cross section in the non-compressed condition.