Pelikan M400 Piston

[Seville]

I picked up this Gunter Wagner Pelikan 400 and have unscrewed the nib (and cleaned) and greased the piston without removing it. Now while the piston is moving freely it only travels about 2 turns on the piston nob while my other 400 does about 4+ turns.
I have been advised and read about how to knock the piston out but my concern is that if it is not fully retracted then tapping on it could blow the piston up. It is a very nice writing vintage Pelikan that I had bought for re-sale but it writes well enough to keep.

Any thoughts?

Philip

DSCN0988_result by Seville36, on Flickr

DSCN0990_result by Seville36, on Flickr

RSCN0991 by Seville36, on Flickr