"Silicone sheet" can be a lot of different products.  Many high
quality silicone products have excellent properties including very
long life and durability.

You can get solid silicone in a variety of hardnesses (designated by
durometer, for softer products usually share A scale).  You can get
sponge in a variety of types with different hardness, different cell
size and different surfaces (smooth or calendered).  Some may be
appropriate for different valve seats such as metal, plastic or wood.

A solid material or a foam will always behave differently under
vacuum than will leather so this needs to be considered especially in
reproducing instruments.  As the vacuum increases, synthetic materials
will behave differently from leather.

Leather may leak minimally at low vacuum but will compress and leak
less under higher vacuum.  It all depends on the nap and other physical
characteristics of the leather.  So a silicone sponge, for example,
might provide a very good seal but this might not be what the original
system was designed for.

You also need to consider how the material sticks to the seat and what
force is needed to release the seal.  A better seal is not always better
if more force is needed to break it loose.

Also, certain silicone products can have a lot of stress in them and
when punched can do nasty things like curl into potato chip shapes.
This is true usually of solid rubber which can have a stressed skin on
the surface.

When I made my E-Roll player system several years ago I chose silicone
rubber for the 3/16" valve seals.  I thought it was the most durable
material and would therefore be the best choice.  It worked very well
but a certain small percentage of the seals warped after punching.
To avoid this problem I switched to EPDM, which is more stable but less
durable, not because I was trying to save money.

Another problem with silicone products is that they are very difficult
to glue in place if this is necessary.  If there is a retaining washer
that holds the rubber down near the mating seat this is probably not
going to pose a problem.

The bottom line is that there are many variable and there is no way to
know what will work without trying it.  Unfortunately it is difficult
to get leather with the same properties as the original material and
modern materials might be needed now but it will require experimentation
to know what will work.

Best regards,
Spencer Chase
Garberville, Calif.
http://www.spencerserolls.com/