Silicone products should never be used in any mechanical musical
instrument, for any purposes whatsoever.

Leather should not normally need sealing, in any application, but if
really necessary I would advise the following.  This was learnt from
Dr Robert Burnett, the late expert on singing bird boxes and is repeated
in Geoff Mason's book on the same subject, and applies equally to
zephyr.

Dilute Evo-Stik contact adhesive with three to four parts acetone and
mix well.  Paint one thin layer onto the material and leave to dry
well.  This glue is the solvent based glue which has a dull yellow
colour and not the PVA based wood glue.  Unused liquid can be kept
in a sealed container.

In the case under discussion, if the leather 'leaks like a sieve'
it is probably too far gone to repair in any way that will survive.
Best to get some decent leather.

Best wishes from GB,  Nicholas Simons