Brakes stick only when it's cold and only once in the morning

So I have a question. My wife has a 2001 Nissan Maxima that has a really peculiar brake problem. Only during the winter (because this problem never happens except December through February) the brakes on her car will seize up first thing in the morning. What happens is that, not the first time you suppress the brake but, the second time you suppress the brake it locks up hard and you have to hold it down until it finally releases and sinks to the floor. This, again, never happens except during winter and only happens the one time in the morning. For the rest of the day she won’t have a problem. Our mechanic thought it was the brake booster so we dropped $400 dollars and replaced it but that did not change anything. He is now stumped and thinks it could be the vaccuum but the pressure is just fine. He said the vaccuum does not seem to be broken but it might be good just to replace it and see if it works. Does anybody have an idea as to what the problem may be? Thanks!

Your pedal goes clear to the floor? You do have a problem. The pedal should never go more than half way to the floor and with power brakes should not go down much more than an inch. As for vacuum problems a simple vacuum gauge check will tell you a lot. Finally - take it to a brake shop!
Good Luck!

Improper wording on my part, the pedal doesn’t sink to the floor just releases to normal suppression. As for the vacuum gauge check, they did that and said there was no problem but they couldn’t figure anything else that it could be. Any other suggestions? I prefer not to take it any other place, my mechanic is amazing, just stumped, and I loyally give him my business first.