The Parker "51" fountain pen was introduced in 1941 and uses a plunger system to draw ink into a clear bladder inside the pen. As with all fountain pens, the ink inside can dry out over time, eventually clogging the mechanism. For this reason, it's a good idea to clean your pen periodically, especially if it will not be used for awhile. The Parker "51" has a patented Vacumatic ink collector that requires a slightly different approach for cleaning than many other pens.
Unscrew the cover of the Parker "51" pen to expose the flexible rubber plunger. Hold the pen tip down over a container, and very slowly squeeze the plunger until a drop of ink appears on the nib. Squeeze the plunger the rest of the way closed.
Release the plunger, wait a few seconds and then slowly squeeze the plunger again, emptying a little more ink. Repeat this process until no more ink drips from the tip.
Wrap the nib with a paper towel, and briskly shake the pen downward, as if you were shaking down a mercury-filled thermometer. This motion removes most of the remaining ink.
Submerge the tip of the pen in cool, clean water, and squeeze the rubber plunger three times to draw water into the pen's reservoir.
Squeeze the plunger slowly to expel the water the same way you expelled the ink. The water will be dark, because some ink still remains in the pen. Wrap the nib in a paper towel again, and shake it downward.
Draw up and expel water several more times until the water runs clear. Allow the pen to dry, and store it or fill it with ink if you intend to use it soon.
If your water is very hard, use distilled water.
Don't use warm or hot water, or you may damage the pen's components.