A medical valve assembly for use in inserting a medical device into a body vessel of a patient, comprising: a tube extending between a proximal tube end and a distal tube end to define a passageway extending along an axis between said tube ends (22, 24); a plunger plate extending radially from said proximal tube end of said tube; a valve housing surrounding said tube about said proximal tube end and extending from a proximal valve housing end to a distal valve housing end; said valve housing including a flange extending radially inwards from said proximal valve housing end and disposed in spaced relationship with said plunger plate to define a distance dimension extending therebetween; an elastomeric seal compressed between said plunger plate and said flange to reduce an inner diameter of the elastomeric seal and establish a closed condition of the medical valve assembly so that the elastomeric seal completely isolates or seals the opening; and, when a medical device is inserted through the opening of the valve housing, the medical device engages the elastomeric seal with an insertion force that is transferred or exerted onto the elastomeric seal, causing the elastomeric seal to axially expand and to move one of the valve housing and the tube relative to the other, wherein the medical device is disposed in engaged relationship with the elastomeric seal to automatically increase the distance dimension between the plunger plate and the flange (3 8) for allowing the inner diameter of the elastomeric seal to increase and establish an open and sealed condition of the medical valve assembly with the inserted medical device, characterized in that the medical valve assembly further comprises a clip slidably disposed over said proximal valve housing end of said valve housing for allowing a user of the medical valve assembly to pre-open said elastomeric seal prior to insertion of the medical device into the medical valve assembly. a tube extending between a proximal tube end and a distal tube end to define a passageway extending along an axis between said tube ends (22, 24); a plunger plate extending radially from said proximal tube end of said tube; a valve housing surrounding said tube about said proximal tube end and extending from a proximal valve housing end to a distal valve housing end; said valve housing including a flange extending radially inwards from said proximal valve housing end and disposed in spaced relationship with said plunger plate to define a distance dimension extending therebetween; an elastomeric seal compressed between said plunger plate and said flange to reduce an inner diameter of the elastomeric seal and establish a closed condition of the medical valve assembly so that the elastomeric seal completely isolates or seals the opening; and, when a medical device is inserted through the opening of the valve housing, the medical device engages the elastomeric seal with an insertion force that is transferred or exerted onto the elastomeric seal, causing the elastomeric seal to axially expand and to move one of the valve housing and the tube relative to the other, wherein the medical device is disposed in engaged relationship with the elastomeric seal to automatically increase the distance dimension between the plunger plate and the flange (3 8) for allowing the inner diameter of the elastomeric seal to increase and establish an open and sealed condition of the medical valve assembly with the inserted medical device, characterized in that the medical valve assembly further comprises a clip slidably disposed over said proximal valve housing end of said valve housing for allowing a user of the medical valve assembly to pre-open said elastomeric seal prior to insertion of the medical device into the medical valve assembly.