The other night I noticed a hairline crack inside the chamber of my 
Hite 128 tenor mouthpiece. I'm concerned it will grow and eventually 
cause the beak to split. What can I use to seal this crack and 
prevent it from getting worse?

Thanks
Mike

I would try JB Weld.   It's the first product listed on this link:
http://www.jbweld.net/products.html which is the slow set type.
I would rough up the area that will be covered by JB Weld with fine 
sandpaper to give better adhesion.  Use a light coat so you won't 
change the mpc tone.  I once fixed a Berg HR mpc using JB Weld and it 
lasted for many years, until I sold it.  It's probably still being 
used today.