Medical device assemblies having a connection mechanism for securely connecting a hub to fluid storage containers in a luer slip relationship are described. An exemplary medical device includes a hub forming a cavity, a second indicating element disposed within the cavity that engages the hub. Additional features of the medical device include a second indicating element contoured to form a line contact with the hub. In a specific configuration, the hub includes a first indicating element attached to the hub and extending proximally into the cavity having a protrusion. In a more specific configuration, the medical device includes a fluid storage container that has an indication system for visually indicating optimal fluid-tight engagement of the hub and the fluid storage container.