too high compression?

Ima thinkin' the vent is there to dump the vacuum that would be trapped in the line to the EGR, if the dump-valve was not there,lol.. If the vacuum got trapped in there, the EGR valve, of course , would not close.On Mopes there might be a lil foam breather on the vent, a lil cube as I recall.

I looked in the catalog once and found a myriad of those Thermo-Vacuum valves. I found several for Mopes spec'd about 10* apart. I was gonna use one to give me automatic advance on my cold 367, to act like a fast idle. (no choke). But with the Dial-back timing delay box, my 367 ended up not needing it. Now I just got to remember to dial the advance back out after the engine warms up!

Good luck on your roadtest.