I recently bought a Sheaffer Legacy Heritage (pictured) and for some reason it leaks from the converter and into the barrel. It doesn't actually leak out of the nib, and I only ever see the ink when I unscrew the barrel but, as I'm sure you can appreciate, this is annoying regardless.
I currently have Mont Blanc Irish Green in there. I'm not sure if it has anything to do with the ink properties, but I don't think it's done this with any other ink before. Another thing I'm thinking it might be is the temperature. I walk to and from school every day, and at the moment (here in the UK), it's extremely cold. In fact - it was even snowing here today (I'll only take one pen to school tomorrow I think and make sure I keep it warm). Could a low temperature make the pen leak a little? It's fine sitting on my desk at the moment. I've written with it and there are no leaks, until I get to school and have a look inside the barrel to find ink over the converter.