A handheld assembly tool system is provided for assembling fluid transfer lines that include a fitting and a tube. In an embodiment of the invention, the assembly tool system includes a fitting fixture for holding the fitting. The fitting fixture includes a flange for supporting the fitting and a selectively adjustable retaining block for securing the fitting against the flange. The assembly tool system also includes a tube fixture for holding the tube and a press device selectively operable to engage and move the fitting fixture such that a portion of the end fitting is forced into the tube.