Back in the day we called them “corks” because they were always natural cork (and fell apart pretty regularly), but today “bumpers” seems to be the preferred term.
Quite a variety of these are available. I for years have mostly been using O ring or drive belt material, but I was feeling not completely happy with the results, and there was too much noise associated with the bumper material on several horns I regularly use.
At the risk of going Scientific American on our readers, there are quite a variety of options out there for bumper material. The article and list over at the Siegfried’s Call website (here) is the most comprehensive I know of, with materials including Buna-N, soft Neoprene, Silicone, and more!
I opted to buy from them their soft Silicone in the .210 diameter, which they describe as follows—
Softest feel with a little bounce particularly with lighter springs. Very little noise, if any. Fairly durable and resistant to oils. Our shop favorite feel.
The first horn to get it was the ASU Chinese Wagner Tuba which had certainly a very hard bumper material from the factory. It made a HUGE difference! The second horn to get the treatment was my own main double horn, which I also felt had valves that made more noise than they should. Again, it made a noticeable difference. Initially it seemed just by a degree, but the more I played it the more my horn felt like a higher end horn! Bumper material does impact your perception of the feel of quality of a horn.
The big picture point being that quiet valves are possible!
I think people these days tend to use bumpers too long that are from materials that tend to harden over time, slowly creating extra noise. Don’t rule out a change of bumper material to reduce your valve noise.