Richard Binder is the Guru of fountain pens. His web site is a treasure trove of information for the fountain pen collector and repairer. I've been collecting and repairing antique fountain pens for over 15 years and and I have three "go to" information sources.
Fountain pens, The complete Guide to Repair and Restoration by Frank Dubiel (known among collectors as"Da Book"
Pen Repair by Jim Marshall and Laurence Oldfield
Richard Binder's website.
Al lthese resources will provide fountain pen makers with loads of ideas for different filling mechanisms as well as solutions to problems of flow.