I am in the midst of rebuilding the valves on a Clark Apollo piano.
Art Reblitz comments that the valve closing against leather is not
recommended in the rebuild because of leakage problems with leather
against leather.  He does not recommend any particular material for
the upper leather valve to close against.  I could use some help
from someone out there who has been through this with the Apollo.
Thanks.

W. Deane Wiley