This was a message from Sakura regarding this problem:
If you come across this issue... before throwing out or returning...
The Pigma Micron pen was originally designed in the early 1980s as a disposable alternative for expensive, refillable technical pens. If you are familiar with technical pens, you’ll know they have to be carefully handled or they will easily leak.
Based on photos we’ve seen and descriptions we’ve heard regarding a leaking Pigma Micron pen, the source is the air vent collar located just below the silver tip of the pen. (See Illustration 1, Area A.) Under normal use, the air flows into the barrel through the vent area. The equalized pressure allows the ink to flow through the microscopic nib structure via capillary action. However, when the pen is unconsciously waved, tapped or spun in your hand while capped, the centrifugal force can cause the ink to come out of the air vent, which is the path of least resistance. Most people are unaware that they have used the pen as an outlet for nervous energy. We’ve seen people use their pen as a drum stick, wave it in their hand or use it as a pointer while talking, tap the pen on their desk while on the phone, unconsciously spin it in their hand, and swing it in their purses and backpacks. Centrifugal force can also be applied to the pen during shipping.
The performance characteristics of Pigma ink require a special ink delivery system within the nib, but the trade off is that the Pigma Micron has to be handled more carefully than an ordinary ball point pen. We pack our pens full of pigment ink. Pigment ink particles are much larger than dye ink particles and therefore more susceptible to gravitational pressure. The pen should also be stored horizontally when not in use.
For a pen that has leaked, the area around the air vent can be carefully wiped clean, but there will also be ink residue inside of the cap.
Take a look inside of your Micron pen cap and you will see a secondary inner cap. (See Illustration 1, Area B.) This 2nd inner cap must also be wiped clean. Otherwise, every time you place the cap back on, the inner cap will transfer more ink to the vent area.
The pen should work fine after both areas are wiped clean.