Not many modern pens use ink sacs. Most vintage pens do. And though a pen sac would seem to be a very simple item, proper pen function depends upon correct sac choice and installation.
If you are starting out in pen repair, a good choice is to start with an assortment of the most common sizes, all straight-sided in profile. The great majority of pens use sacs in sizes between 14 and 18 (it is usually stated that the number indicates outer diameter in 64ths of an inch; this is not entirely correct, however, as it actually indicates the diameter of the hole into which the sac is an easy slip fit). Tapered sacs are needed for only a handful of models, and necked sacs can be dispensed with entirely. In those rare cases where the section nipple is so small that a sac that properly fills the barrel will not stay on, the nipple can easily be built up by adding a short piece cut from another, smaller sac, over which the sac can then be attached as usual.
There are references that list the "factory correct" sacs for various pen models. They can be useful, but don't rely on them blindly. A sac should be an easy sliding fit into the barrel -- let that be your final guide. If anything, err on the small side. Trying to maximize ink capacity is a trap many fall into. Sacs that are too large, even by a little, are more prone to binding on the pressure bar, while tight contact with the barrel walls allows body heat to be transferred more easily to the sac's contents -- not desirable, as warming of the air in the sac can lead to heavier ink flow and even dropping of ink. In fact, one of the most common causes of blobbing is a sac that is too large. This is especially the case with oversized pens, since sacs in the largest sizes (22 and up) are often insufficiently stiff to resist deformation, whether from internal vacuum or the sloshing of the ink.
For sorting out sacs by size, use a fractional inch drill gauge -- typically a steel plate with measuring holes from 1/16 to 1/2 of an inch in 1/64 inch increments. Try out latex or PVC sacs with a gauge and you'll find that there is a fair amount of variation from sac to sac. You will probably also find that this variation is enough that a 1/2-size increment is meaningless (that is, there is no meaningful difference between a 19 sac and a 19 1/2). And 1/64 of an inch (= 0.40mm) is small enough that if you don't happen to have the exact size of sac on hand, going down one full size (16 for 17, for example) should do no harm.
In most cases sacs will have to be cut to proper length. Again, cut to measure, rather than blindly following a reference chart. Just as one should allow a bit of space around the sac, so should a bit of space be allowed at the end. This is particularly important with button-fillers. Small sharp scissors make neat sac trimming easy. Before attaching a new sac, take a second to check for pinhole leaks. Pinch the opening shut and give the sac a quick squeeze to see if it remains inflated.
The traditional adhesive for attaching sacs is shellac. You may see that some people use rubber cement or nail polish. Both are much inferior to shellac, however, and can do serious damage to plastic sections. Shellac should be allowed to dry thoroughly before the pen is used. For silicone sacs, we have found that a stronger and more reliable seal is provided by a silicone sealant such as Devcon Silicone Adhesive 12045 (Loctite 908570 is another brand successfully used by customers). Although old-time pen repair kits typically included a sac-stretching tool, in most cases the sac mouth can be stretched over the section nipple with finger pressure alone. The one application where a sac stretcher does come in handy is where the sac has to fit over a long breather tube -- Skylines are the most common instance of this.
Sacs should always be dusted lightly with pure talc before the pen is reassembled. Medical-grade unscented talc can be found at pharmacies. Talc dust should not be inhaled: read the warning labels. Talc acts as both lubricant and protective layer, prolonging the life of the sac while also giving a measure of protection to the pen's barrel protection from the sac (more on this below).
Thin-walled sacs are an unnecessary solution to an imaginary problem. Sheaffer and other makers of pneumatic-filling pens thoroughly tested their components for optimal functioning, and used standard-thickness sacs exclusively. Read more about sac selection for Touchdown fillers here.
The standard material for pen sacs has long been latex rubber. Other materials currently in use include PVC (vinyl) and silicone. Each material has its pros and cons; these are summarized in the following chart, and explained at further length in the text below.
Latex is still an excellent all-around choice, with only two significant disadvantages. Whether due to changes in the rubber formulas used, or -- more likely -- to the increased popularity of specialty inks (more on inks for vintage pens here), premature failure of latex sacs has become more common, with new sacs sometimes turning soft and gooey in a matter of months or even weeks. The more serious issue, however, is that when latex rubber breaks down, it releases highly reactive sulfur compounds that can permanently darken celluloid and other permeable plastics. This is particularly of concern with button-fillers, since their barrels are tightly sealed leaving sac deterioration byproducts no way of escape.Silicone rubber is virtually immune to these problems, as it is largely inert and highly resistant to chemical action. It is less stretchy than latex, but springier, and retains its shape even when left compressed for years. Silicone's main weakness is its relative permeability to air and water vapor. Ink will dry out more rapidly in an unused pen with a silicone sac, and it is advisable to leave a silicone-sacked pen with its nib elevated when not in use to prevent gradual ink seepage into its feed. Silicone is not as tough as latex, so should not be used for high-stress applications such as twist-fillers and Vacumatic diaphragms. Pressure bars with sharp edges are more likely to tear a silicone sac than one made of latex.
The other material commonly used for pen sacs is PVC (vinyl). The transparent "Pli-glass" sacs used by Parker in the 51 Aerometric were PVC, and many if not most are stained but still serviceable after decades of service. PVC is still a good sac material, but has been known to cause severe damage when left in prolonged contact with celluloid and other soft plastics. We recommend it be used only in pens with a metal sac housing that prevents direct contact of the sac with the inside of the barrel.
NOTE: Many sellers describe their transparent sacs as silicone, yet in nearly every case the sacs are actually PVC. If the sac is to be used in a celluloid pen, it is vital to know the difference. This video shows how you can tell the materials apart.