Pat, for another method, try this.  I have a small heat gun that
I use for detail work, such as bending hammer shanks. The nozzle
is about 1/2-inch diameter and it comes with an adapter that cuts
it down to an even smaller 1/4-inch or so.

I can point it at the edges of the metal and away from the center.
Then, when the shellac starts to soften, I use needle nose pliers
to pull the valve seat off.  Try it, it works.

Jeff Davis in Seattle