Generally factory fibers are significantly more brittle if you've used the sight for any length of time outside. The fiber acts the same as any other composite will and hardens and degrades with time and UV exposure. This leads to a brittle fiber. Replacement fibers of any quality will do the same thing after several years in the field, also. There's simply no such thing as a lifetime fiber.
As for tips on working with fiber optics....
Brittle fibers MAY become a bit more flexible with some gentle persuasion from a blow dryer or very gentle using a heat gun. There's a point (temperature) at which it will harden more that you don't want to go above, though, so you have to keep the heat a long ways away and be careful when trying to get the brittle fiber back into the pin. Burning the end to create the mushroom that keeps the fiber in the pin hole is tougher with brittle fibers as well. The brittle fiber will hold its shape longer when next to the flame and then collapse in a puddle all at once, so it may take a few tries to get the end properly melted on an old fiber. Old, brittle fibers are best off being replaced, in my opinion.
Working with new fibers is quite a bit easier. They are much more receptive to bending around pins without breaking. You can push and pull on them some to get them fit into the sight without doing any damage. Once you get the fiber in place and ready to melt the end to mushroom it out so it can't slip back inside the pin body, then you simply use a tiny flame and move it closer to the point of the fiber until the fiber melts then quickly move the flame away. A fiber will naturally melt into a circle, so you don't have to "tamp" it with your finger like you would when singeing the end of a poly rope. If you get it too hot, then more of the fiber will melt, and the mushroom will be larger than you need; simply cut it off and try again. A larger mushroom equates to the fiber covering more of the target and a lower quality (less focused) light transmission. A mushroom that is just big enough to hold the fiber in the pin will cover the least amount of the target and will have better focus for the light (i.e., pin-point precision versus diffused light); therefore the pin will appear brighter. Remember the fiber will melt well before the flame ever touches it as you move the flame closer to the pin; be ready to pull it back quickly, and you should be able to get good results in the first attempt.
One thing to consider is that a Bic lighter generally has a "dirty" flame, so you will probably get better results for pin brightness if you use an alcohol burner since there will be no impurities to settle into the pool of molten fiber as it cools. The amount of this difference is so miniscule that you may or may not see any difference in your pins in the field. Don't worry about this too much, but if you have an alcohol burner available, then use it.
Another thing to watch for when working with fibers is cracks. If you see a bright spot in the middle of a fiber, then that indicates a crack occurred. The light will leak out in that place, and the fiber will act like you placed a mirror in the middle of it. The light transmission will be half or less through that fiber with a crack, and it's time to replace it if you want the brightest pins your sight can get.
Hope this helps a little.
