You might have a valve leak that causes hissing when the pneumatic
is activated.  The porous material over the valve port was there to
prevent debris (ie dust, tiney insects, mouse t*rds etc etc) from
getting between the upper valve face and the seat.  You said you
recovered the pneumatic, but did you also service the valve?  A quick
try 2 fix is to hold down the valve with a voicing needle, (or tooth
pick) and apply the vacuum cleaner rigged to a 1/2 inch or 3/8 inch
tube so you can get at it.  If the top valve seat disintegrates as a
result of this operation, be glad it got sucked into into your hoover
and not into your simplex.  If this happens, pull out the pneumatic and
go to work on the valve cover. in order to get ot off.  Yes that is
shellac that is sealing it in.  If the vacuum trick does work, cut off
a piece of an old wool blanket or sweater, and glue over the valve
opening to prevent deja vous.

Richard Moody