I agree with Mr. Spencer Chase about the sponge rubber.  I've been
using epichlorohydrin sponge rubber on valves for years.  I get it
from McMaster Carr and it isn't cheap.  It has a durometer hardness
of 3 which is incredibly soft and resilient.

I have a scrap of it that is maybe twelve years old and it is as soft
and supple as the day I bought it.  It is 100% airtight, as opposed to
leather, and even if it didn't last as long as leather it surely will
last at least as long as other rebuilding products.

Examine a Gulbransen valve when the stem is pulled from the valve --
it was well coated with a clear rubber of some sort.  Combined with its
efficient minimal use of vacuum by design, it is really no secret why it
was #1.

John Meadows
Oil City, Pennsylvania