I'm gradually developing a strong distaste toward any pen with a rubber sac. I've just had too many problems with them. I'm beginning to understand why a "sac-less" design was such a big selling point back in the 1930s and 1940s.
I've seen "new old stock" pens that came with rubber sacs 40+ years old and worked Just Fine. I've also, unfortunately, seen brand new rubber sacs that failed after a relatively short period of light usage.
I've got a suspicion that some inks (i.e. some more than others) cause rubber to age and deteriorate.
I've got a suspicion that some sacs available today are not the greatest quality, and perhaps it may vary from batch to batch.
My latest mishap is a failing rubber bulb in my Edison Glenmont bulb-filler. It has't ruptured yet, but it has become limp and soft, and it has a small pimple-like swelling on the side of it. It clearly is in need of replacement. The pen is a few years old, and I don't think I've used it that hard.
One of my friends -- who does quite a bit more writing than I do -- has gone through multiple sacs in his Sheaffer Snorkel over the last few years.
I've had distressingly bad luck with my favorite pens lately. First I had two TWSBI Vac 700s which I loved in almost every way until they each suffered from (different) broken plastic parts. Now the Glenmont is letting me down. True, it's not a difficult or expensive fix, but... Are there not designs without this vulnerability?
Has the rubber sac outlived its time?