It only happens in very cold weather.
It goes away after several tries and becomes normal.
Do I have to do anything, or just live with it?
Thanks.

 

Well, seeing as you live in michigan (pretty close) and I live here in Toronto and it is about (minus) - 22° right now, I think the whole car is stiff.

Stop worrying !! lol

 

When was the last time you changed the brake fluid? Sounds like water in the fluid is forming ice.
Just an idea of where to start from

 

I haven't done it since I bought my car 5 years ago. That's something I plan to do.
The pedal becomes so stiff that it feels like no power boost at all.
It might be the icing in the brake line.
Thanks.

 

Stiff in the morning, isn't that normal?

It isn't ice in the brake line. It is likely that your car has not had the brake recall done. The check valve in the line to the brake booster is not holding vacuum. Your hoses near the brake booster and the rubber hose that attaches to the intake manifold should have small white dots on them. If they don't get the recall done, that will fix your problem.

 

You dirty old man ...lol...