So the air loss must be internal, but where
There are "walls" inside partitioning off the the required chambers.
I've cut my own gaskets using blotter paper (is it still available?)
And I am fairly certain I've seen reproduction gaskets (try Train Tender)
I have never used a putty or other sealant. I also wonder whether you have excess sealant inside that has altered the characteristics of the chambers and passages, killing the sound.
