Hello there, I got a little problem with a Pelikan M400 (1980s model). The piston seal has detached from the piston rod and is not sticking in the barrel. Thus, I have to disassemble the M400. Question now: How can I disassemble the piston unit from the barrel? Is that friction fit with the M400 just as it is with the vintage 400? Greetings and Thanx