Gary,
You say your o-rings are Viton. This is interesting since the only Viton o-rings I'm familiar with were a brown material. Yours are Black, which would lead me to believe they are just standard Buna N (nitrile) O-rings. Which has a -35 to 250F rating.
Not that it really matters, I'm sure that's all that the rubber gasket was.
Where did you get the O-rings from? All I'm getting at is that I've never knowingly seen or used black Viton O-rings before.