In a pen that has been stored for a long time, an irregular ink flow is nearly always a question of cleaning rather than of the nib. Dried ink encrusts the fine channels of the feed over the decades, and even a night in a glass of water does not always dissolve such deposits completely. Repeated flushing with lukewarm water is the answer, with a drop of washing-up liquid if need be, until the water stays clear. Hot water should be avoided, as it attacks the feed and the seals.
If the flow is still too sparse afterwards and the slit looks very tight, it can be opened carefully by easing the two halves of the nib a fraction apart with the fingers. The smallest of adjustments will do, and the tips must afterwards sit at exactly the same height and meet cleanly. Pliers are not needed for this and readily do permanent damage to an old nib. If neither step brings results, the cause often lies in a hardened piston seal or a distorted feed — both of which a specialist workshop can put right.