Olds 403 EGR valve black/yellow check valve.

[BlueBaron762x39]

I have a new EGR valve for my 1979 Olds 403.  However, after looking at the vacuum diagram and other information online, it looks like I am missing black/yellow check valve of some kind that is supposed to be placed in the vacuum tube between the EGR valve and the carburetor.  I'm searching online all over the place and have been unable to locate this part anywhere so far.  Is it essential to the operation of the EGR valve to have this black/yellow check valve? 

[Wallington]

#560111 / #419459 depending on model/year. It's actually a Thermal check-valve and sits under the steel cover plate on top of the water pump plate/engine block. As it warms up with the engine, it eventually cuts vacuum to the EGR. Many wouldn't even have it at all, but Californian cars may be an all or nothing situation.
I think TV-202 by Standard may be a similar alternative, if available.

GM 560111 Olds 403 EGR valve NOS (1) by Aus78Formula, on Flickr