The Advanced Pressure Valve is required to craft the Gas Mix Compressor, but doesn't appear to have a recipe of it's own. Might also be missing in 1.9.x too.

I have no idea what your link is referencing

Errr, sorry... that was meant for a different bug report. I'll check the recipe, it should be there...

Do you have enderio installed as well?

yes, it's the last 1.8.9 build before they switched to 1.9.4 builds
EnderIO-1.8.9-3.0.0.26_alpha.jar

Ok there seems to be a bug with the EnderIO and IC2 versions of this recipe, and if either of those are present the vanilla recipe is disabled - so it becomes uncraftable.

Will push a fix in a minute.

Great, thanks :D

Should be fixed in this build: http://minecraft.curseforge.com/projects/advanced-generators/files/2304228