A needleless access connector for connecting medical devices, the connector comprising: a housing comprising a male Luer fitting at one end; and a female Luer fitting at the other end; and a collapsible valve disposed in the housing , the collapsible valve having a first axial length from 15.75 to 20.83 mm and comprising: a first portion having a cylindrical shape with at least one smiley cut in a section thereof and having a second axial length of less than 6.35mm, wherein the first portion of the collapsible valve creates a seal at an opening of the housing ; and a second portion having a hollow, cylindrical shape and being coupled to the first portion , the second portion having a third axial length and an annular outside surface over the entire third axial length, wherein the annular outside surface spans 30% to 66% of the first axial length of the collapsible valve , wherein the collapsible valve is configured to provide a positive displacement when actuated by a male Luer inserted into the opening , and wherein the housing and collapsible valve are configured to provide a positive displacement and a priming volume that is from 0.17 to 0.19 milliliters. a housing comprising a male Luer fitting at one end; and a female Luer fitting at the other end; and a male Luer fitting at one end; and a female Luer fitting at the other end; and a collapsible valve disposed in the housing , the collapsible valve having a first axial length from 15.75 to 20.83 mm and comprising: a first portion having a cylindrical shape with at least one smiley cut in a section thereof and having a second axial length of less than 6.35mm, wherein the first portion of the collapsible valve creates a seal at an opening of the housing ; and a second portion having a hollow, cylindrical shape and being coupled to the first portion , the second portion having a third axial length and an annular outside surface over the entire third axial length, wherein the annular outside surface spans 30% to 66% of the first axial length of the collapsible valve , wherein the collapsible valve is configured to provide a positive displacement when actuated by a male Luer inserted into the opening , and wherein the housing and collapsible valve are configured to provide a positive displacement and a priming volume that is from 0.17 to 0.19 milliliters. a first portion having a cylindrical shape with at least one smiley cut in a section thereof and having a second axial length of less than 6.35mm, wherein the first portion of the collapsible valve creates a seal at an opening of the housing ; and a second portion having a hollow, cylindrical shape and being coupled to the first portion , the second portion having a third axial length and an annular outside surface over the entire third axial length, wherein the annular outside surface spans 30% to 66% of the first axial length of the collapsible valve , wherein the collapsible valve is configured to provide a positive displacement when actuated by a male Luer inserted into the opening , and wherein the housing and collapsible valve are configured to provide a positive displacement and a priming volume that is from 0.17 to 0.19 milliliters. <EOS>