Jim Mamoulides, a regular here, has a profile of this pen and its 'relative' the Sheaffer Craftsman on his website
http://penhero.com/PenGallery/Sheaffer/SheafferTipDip.htm. I am not certain about the repair process on this particular pen, but it will definitely require disassembling the pen and attaching a sac to the appropriate part with an adhesive, usually shellac. Sacs can be purchased from Woodbin (
http://simcom.on.ca/woodbin/) online or the Pen Sac Company (
http://www.pencentral.com/pensac.htm) or perhaps some other places.
Alternately, a qualified repair person could do the work for you for a small fee (usually $15-$30). People often mentioned in connection with pen repair are Dennis Lively (
http://virtualinfamy.com/penden/), Sherrell Tyree (
http://www.ink-pen.com/), Aaron Svabik (
http://www.pentiques.com), Richard Binder (
http://www.richardspens.com) and many others. See
this thread for more discussion of repair people for Sheaffer pens.
These Touchdown fillers are quite reliable, often needing only a sac replacement to be in perfect working order, if that. I'm sure others here can provide you with more details on sac size and repair process.