I am a user rather than a mechanic. I have a 1960s Parker English Duofold (same size as Junior but marked Duofold). I bought it as a user grade pen a while back and it has been fine until ink started leaking from the bottom of the sack.
Having done some FPN research,with great care and no difficulty I took it apart and found everything as expected. Until I realized there still seemed to be something in the ink sack and a tatty, thin, plastic rod fell out. This piece is solid, too large to fit in the breather tube, one end appears to have been snipped with pliers and the other looks to have some sort of residue.
The pen has clearly been re-sacked at some time (plain black rubber) and my guess is that this plastic rod was used for applying shellac and somehow got left behind (rather like surgeons and scalpels!)
I have attached a picture. Can anyone shed any light?
Thanks in advance, Martin