I just received a nice MB 334 1/2 and I have a very curious problem: I haven't been able to ink it up. The problem seems to be that the piston is stuck in the upward position. Since it's a vintage pen I do not want to force the piston.

So the question is: what can I do? I thought maybe I could soak the pen for some time or put it a couple of minutes in the ultrasound. Apparently only the cap and the blind cap are BHR so maybe this could be an alternative.

Any ideas?