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 a cavity of the housing , the collapsible valve having a first axial length from 15.75 to 20.83 mm and comprising: a cylindrical first portion with at least one arcuate cut in a section thereof and having a second axial length , wherein the first portion of the collapsible valve creates a seal at an opening of the housing ; and a cylindrical second portion being coupled to the first portion and having a void , a third axial length, and an annular outside surface over the entire third axial length, wherein the annular outside surface spans 56% of the first axial length of the collapsible valve, wherein the collapsible valve is configured to provide a positive displacement when collapsed by insertion of a male luer into the opening such that a fluid is allowed to move around the valve and between the male and female Luer fittings, and wherein the housing and collapsible valve are configured to provide 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 a cavity of the housing , the collapsible valve having a first axial length from 15.75 to 20.83 mm and comprising: a cylindrical first portion with at least one arcuate cut in a section thereof and having a second axial length , wherein the first portion of the collapsible valve creates a seal at an opening of the housing ; and a cylindrical second portion being coupled to the first portion and having a void , a third axial length, and an annular outside surface over the entire third axial length, wherein the annular outside surface spans 56% of the first axial length of the collapsible valve, wherein the collapsible valve is configured to provide a positive displacement when collapsed by insertion of a male luer into the opening such that a fluid is allowed to move around the valve and between the male and female Luer fittings, and wherein the housing and collapsible valve are configured to provide and a priming volume that is from 0.17 to 0.19 milliliters. a cylindrical first portion with at least one arcuate cut in a section thereof and having a second axial length , wherein the first portion of the collapsible valve creates a seal at an opening of the housing ; and a cylindrical second portion being coupled to the first portion and having a void , a third axial length, and an annular outside surface over the entire third axial length, wherein the annular outside surface spans 56% of the first axial length of the collapsible valve, wherein the collapsible valve is configured to provide a positive displacement when collapsed by insertion of a male luer into the opening such that a fluid is allowed to move around the valve and between the male and female Luer fittings, and wherein the housing and collapsible valve are configured to provide and a priming volume that is from 0.17 to 0.19 milliliters. <EOS>