A pen that has started skipping after a few months usually has not broken; it has a feed channel half-closed with dried dye and a skin of surfactant residue. Plain water will eventually shift that but can take days of soaking. This clears it in about an hour.
It is a mild solution of household ammonia and a non-ionic surfactant, supplied as a concentrate: one capful (about 3 ml) into 100 ml of cool water. Draw it through the pen with the converter, leave the nib unit standing in it for an hour, then flush three times with clean water and dry nib-down.
Two warnings, both learned the hard way. Do not use it hot — heat and ammonia together will soften some adhesives. And do not use it on a pen with a celluloid barrel or any vintage pen with shellac-set internals; for those, cool water and patience only. 60 ml, with a blunt-tip syringe and a measuring cap.
