PISTON STOPPER AMAZING EYEDROPPER:
Eyedropper-filler" is not just a term of speech - some very high-quality fountain pens, such as the Danitrio Mikado and the Junikaku series pens, and even some Nakaya, Nimiki models utilized the entire pen barrel as an ink reservoir and are most conveniently filled using a common high capacity ink dropper.
The eye dropper has a piston mechanism that helps you to intake a huge amount of ink in the barrel and when you almost finish using the ink, the piston will help you to drain out rest of inks when you clean your pen. This mechanism creates an airtight atmosphere to ensure leaking out of ink and also constant ink flow to the feeder.
The piston mechanism is the most common mechanism in vintage non-cartridge pens. Historically, it seems to be much more prevalent in Europe than in the US. German pens are usually piston fillers, including Pelikans and MontBlancs. Playing with the screw at the end of the pen moves a plunger down. Playing with it the other way moves it back up, drawing in ink. The ink is stored directly in the barrel.
BULB FILLER AMAZING MECHANISM:
The bulb-filler, which might better be termed the bulb-and-breather-tube filler, is the most basic form of a filling system with numerous more complicated variants.
These pens are filled by repeatedly squeezing the bulb. With each squeeze, air is expelled through the breather tube and out the feed. Each time the bulb is released, ink is drawn in. This is one of the simplest and most elegant filling systems, whose only real drawback is a slight awkwardness in emptying. This entails repeated slow squeezes of the bulb, expelling the ink from the nib and feed drop by drop.
Prominent examples of bulb-fillers include the Postal, various economy-line Wahl-Eversharps (a Bantam is shown above), and the French Stylomine. Most pump-fillers, including Vacumatics, Dunns, and Ink-Vues work on the same principle, either adding a device to squeeze the bulb or substituting some other means of intermittently pressurizing the barrel (typically, a reciprocating plunger).
LEVER FILLING MECHANISM:
The lever filling fountain pen was presented in 1908 and Sheaffer was the first company to successfully market it. Most pen makers adopted the lever fill soon thereafter, where it remained dominant until the post war era.
The lever filler is very simple in operation. A lever on the outside pushes a springy bar on the inside. The bar compresses the rubber ink sac. When the lever is released, the ink sac reinflates, drawing ink into the pen in much the same manner as an eyedropper.
Assuming the lever is intact, lever fillers are very easy to repair if you can get the section off. You'll have to replace any crumbled ink sac (simple enough for trained chimpanzees to do) and possibly a pressure bar if the existing one is snapped or rusted. The hard part is in the initial disassembly.