A medical valve assembly for use in inserting a medical device into a body vessel of a patient, comprising: a tube extending between a first tube end and a second tube end to define a passageway extending along a longitudinal axis between said ends (22, 24); a plunger plate extending radially from said second tube end of said tube; a valve housing surrounding said tube about said second tube end and extending from a first valve housing end to a second valve housing end; said valve housing including a housing flange extending radially inwards from said second valve housing end and disposed in spaced relationship with said plunger plate to define a distance extending therebetween; an elastomeric seal compressed between said plunger plate and said housing flange and having an inner diameter for establishing a variable seal of the medical valve assembly; a compression member disposed within said valve housing and biased against said plunger plate for decreasing said inner diameter to establish a closed inner diameter of the elastomeric seal and a respective closed condition of the medical valve assembly; a pair of lever arms pivotably connected to said valve housing and radially compressible to overcome the bias of said compression member and effectuate axial movement of said valve housing relative to said tube to increase the distance between said plunger plate and said housing flange and increase said inner diameter of said elastomeric seal from the closed condition; and a locking ring rotatably connected to said second valve housing end and rotatable relative to said pair of lever arms from a closed position wherein said locking ring is disposed in spaced and non-engaging relationship with said pair of lever arms to establish said closed condition of the medical valve assembly to an open position wherein said locking ring engages and radially compresses said pair of lever arms to establish and maintain an increased open inner diameter (D o) of said elastomeric seal and a respective open condition of said elastomeric seal; and wherein said locking ring is rotatable to at least one intermediate position disposed between said open and closed positions to partially radially compress said pair of lever arms and establish at least one intermediate inner diameter (D 1, D 2) of said elastomeric seal being greater than said closed inner diameter of said elastomeric seal but less than said open inner diameter (D o) of said elastomeric seal. a tube extending between a first tube end and a second tube end to define a passageway extending along a longitudinal axis between said ends (22, 24); a plunger plate extending radially from said second tube end of said tube; a valve housing surrounding said tube about said second tube end and extending from a first valve housing end to a second valve housing end; said valve housing including a housing flange extending radially inwards from said second valve housing end and disposed in spaced relationship with said plunger plate to define a distance extending therebetween; an elastomeric seal compressed between said plunger plate and said housing flange and having an inner diameter for establishing a variable seal of the medical valve assembly; a compression member disposed within said valve housing and biased against said plunger plate for decreasing said inner diameter to establish a closed inner diameter of the elastomeric seal and a respective closed condition of the medical valve assembly; a pair of lever arms pivotably connected to said valve housing and radially compressible to overcome the bias of said compression member and effectuate axial movement of said valve housing relative to said tube to increase the distance between said plunger plate and said housing flange and increase said inner diameter of said elastomeric seal from the closed condition; and a locking ring rotatably connected to said second valve housing end and rotatable relative to said pair of lever arms from a closed position wherein said locking ring is disposed in spaced and non-engaging relationship with said pair of lever arms to establish said closed condition of the medical valve assembly to an open position wherein said locking ring engages and radially compresses said pair of lever arms to establish and maintain an increased open inner diameter (D o) of said elastomeric seal and a respective open condition of said elastomeric seal; and wherein said locking ring is rotatable to at least one intermediate position disposed between said open and closed positions to partially radially compress said pair of lever arms and establish at least one intermediate inner diameter (D 1, D 2) of said elastomeric seal being greater than said closed inner diameter of said elastomeric seal but less than said open inner diameter (D o) of said elastomeric seal.