I recently changed my brake pads with some new ones that have been in my shed for years. They are not packaged, but as I am in the UK, which has a very variable climate. The shed gets very hot and humid in the day, and cools down at night.
Does this have an effect on the brake pads, with V brakes?
Almost any rubber product will degrade over time, loosing its properties. Different rubber compounds will degrade differently, depending on the formula and method of fabrication and, of course, environmental factors. This applies to brake pads, tires, grips, valve seals, inner tubes and more.
Usually brake pads tend to "dry" over time, turning less flexible and prone to cracks. A less flexible pad is less capable of creating friction. I know of some rubber (or rubber-like) products that when degraded turn to a thick paste that will stain fabrics, skin, etc. but never seen a brake pad degrade this way, if that where the case, it would not last eve one braking.
To test a V-Brake pad press hard with your fingernail on its working surface. You should be able to sink the fingernail just a little bit, leaving a pressure mark that will shortly disappear. If it feels hard, wood-like, almost sure you are better discarding it.
However, pad age is not the only factor that can degrade braking performance. Dirt and grease on pad or rim for example. I recommend washing them with liquid dish washing soap. Another factor is calibration. Brake pads not properly aligned may will not perform as designed.
Finally, there are different rubber formulas for aluminum or steel rims, (And for carbon fiber I guess) and I have tested pads that work wonders on steel are worthless on aluminum. Also have observed that the same pad can perform differently on aluminum rims made of different alloys.
<< End of answer and begin of anecdote >>
Recently I committed the mistake of trying to slowly descend a long pronounciated slope with a road bike from 1984 that still had the original pads (The ones it was originally sold with). The result was the friction generated was so poor that they only caused the rims to heat up to the point that both inner tubes were blown out. One rim was steel and the other a single wall aluminum.
Since V-brakes use pads that are almost exclusively made of rubber or a rubber-like compound, they will have experienced some changes due to the temperature and moisture fluctuations. The rubber is most likely dried out and/or cracked and significantly harder. What does this mean for your braking ability? It means the pads won't have much grip and will instead slide against the rim. Plus, if any cracks begin to separate while riding, parts of the pad may shear off and your braking ability will quickly degrade. Since v-brake pads are relatively cheap you should invest in a new set and avoid any trouble.
Also, if the pads had been in a sealed package, they would still degrade, just much slower.
Aaron is correct. Rubber by nature will age in any environment. Even buying new is no guarantee you will get full life from rubber products. If at all possible, check manufacturing dates before purchase. Four years (shelf/use) is about the limit for effective performance. I do tend to keep the old ones laying about for a few years after replacements have been mounted, but have not yet been guilty of putting old ones back on an active bicycle.
I recently had an emergency. I was checking my bike over before loading on car & discovered a very worn v-brake pad. Didn't have any spares on hand. Looked through old parts & found old V-brake (about 12yrs old) with pads still attached. Pulled the pads. Pads were worn but had plenty of rubber left & did not feel hard. Put these on my bike & went for 2 hour ride. Brakes worked fine. While age might eventually cause a pad to perform poorly, it will probably take a really long time. Also,, while I've cleaned & removed embedded bits from by current road bike's pads, I've never replaced the pads after many years of service (seldom need to use brakes on most of my road rides). So,, I'd use an old package of "new" pads without hesitation.
In my case, I have old Shimano BR-MC30 and what I did after many YT videos and reading forums, use sand paper on the pads and clean the wheel surface with alcohol. My bike was stored for many years and at some point never used (I think more exactly 10 years+)
