Thanks for the advice.
I took a Majestic rollerball that works fine, and replaced the cap with another Majestic kit. Here is where it gets interesting. The Majestic kit with the issue is one from Rockler. The true PSI kit HAS #$%@*& different threads for the cap screwing to the body (but the threads for the fountain pen/rollerball part are exactly the same)! I had no idea they were different kits, but now I do.
Having said that, I am wondering know if the "stud" in the top of the cap that keeps the nicely cast head piece on the cap on is longer inside the pen cap than it needs to be? It looks like a piece of brass that has been "riveted" into the top, and may be long (what do I know though....)
Ed, I do not know how "far" into the feeder piece this needs to be, I wish I was better with the whole fountain pen nib stuff. I will look at a few of the kits I have and try to make a judgement call, thanks for thinking of it. I tried the rollerball in it as well, and it was too short for that as well..........EEERRRRRRGGGGGG!