As Bob Taylor says, it all has to do with the quality of the original
product.  I believe the 40-year-old Dynavoice valves and maybe the
pouches are sponge neoprene.   (I sent one to D. L. Bullock last year
and I believe that's what he thought they were made of).  I had very
few torn pouches, but they were not brittle, all the valves were still
supple.  I did reline the exhaust port faces of the valves with gum
rubber suggested by Mr. Bullock, but more to reduce travel rather than
because of the face being degraded.

Tom Lear
San Francisco CA