There are actually two more things needed to finish the vacuum booster. I broke the retaining clip that locks the valve action mechanism, and will have to make a new one.
Also, there should be a tiny little rubber boot here, to stop dirt getting into the valve shaft. I have NEVER seen one of these, even a rotten one. I have seen some attempts to make do with a crude canvas replacement, but i'm no fan of that, so i'll head off to the local toy shop during the week and see what thay have for R/C vehicles that can be used here.