Thanks to D. L. Bullock for the update of the Dynavoice valve material.
I guess I was working from old mis-information.  When I first contacted
Mr. Bullock he asked if the valves might be made of sponge neoprene.
Later I sent him a valve to look over, but didn't think to verify the
true nature of the material.

Regarding valve travel:  The gap between the pouch and the valve stem
on the Dynavoice is usually less than 1/32".  However, the movement
between the exhaust port and the valve was, as Mr. Brougher says,
excessive and is hard on the pouch and inefficient.  The answer is
to reline the exhaust port face of the valve with 1/32" gum rubber
punchings (I got mine from Mr. Bullock).  I find that my valves work
perfect with that addition.  I'm sure glad to know that the pouches and
valves are all gum rubber and so will probably last another 35 years.

D. L. Bullock comes through again!

Tom Lear
San Francisco CA