Dole 1933 replaced with a Gorton # D as main Vent

Cast Iron

So after reading Dan's books and visiting the wall, I replaced the Dole 1933 on my 2"X 18' main with a Gorton #D. The main now fills with steam very quickly, in about 3 minutes from when the header gets hot.

Here is my question, although the main and return are hot (approx 210* according to my infra red thermometer) the Gorton does not get hot enough to close. Is this normal? I suspect that once the main fills with steam, the steam then goes off into the risers, etc. Which is why the Gorton never gets hot enough to close. After quite a while,I think when all the rads are hot, the Gorton will close.