Dole 1933 replaced with a Gorton # D as main Vent

Cast Iron

So after reading Dan's books and visiting the Wall and getting some advice, 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 dry return are hot (approx 210* according to my infra red thermometer) the Gorton does not get hot enough to close initially. The Gorton is located right where the return turns downward to go back to the boiler. 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.

I was just wondering if this made sense. Thanks and Happy New Year.