In regard to old style Ampico valve blocks, when I take these apart
I always seal the bottom seat with fresh shellac, especially if these
are wood.  They can sometimes be porous and allow air to seep through.
I found that it is even good to do this when there are Bakelite bottom
seats, as they are sometimes loose.  I check these and reglue them when
I find that condition present.

Another thing which is totally necessary is to shellac the entire valve
block except for the gasket area. I use a thicker shellac for this, just
as the factory did.  I hope one of these answers will help you.

Jim Jelinek