Old hard rubber pens, especially eyedroppers, can be a difficult repair because the pens are so old and frail. It is very easy to overtorque them and crack or crush them.
The problem, especially in eyedroppers, is that old ink effectively turns into glue on these old pens. Eyedroppers are so problematic because the ink always seeped into the threads that held the section to the hollow barrel that served as the pen’s ink reservoir. Other pens with ink sacs get ink-glued when the old sac gave out flooding the inner barrel with ink.
Lucky for you, the solution is really simple. Once again H2O comes to the rescue. Fill a cup with room-temperature water and soak the pen over the line separating the section from the barrel. Let it soak for 12 to 48 hours. This is usually enough time to loosen the old ink and allow the pen to open the way it should.
Sometimes it takes a little heat. Heat is the enemy of your old hard rubber pens. Open flames will melt or burn the pen very quickly. Hot water will discolor the pen, too. If you need the heat, just hold the pen briefly under warm to hot water flowing from your kitchen tap. Don’t expose the pen to the heat for more than a couple seconds, and keep an eye out for discoloration. It doesn’t take a lot of time or heat to start the discoloration process.