Why would your brakes freeze on a very cold morning?

My wife has had this same problem with her 2001 Subaru legacy. Last night she drove a friend's '98 legacy and got into an accident because his brakes would not engage. She reports that the brakes were stiff, would not budge and could not stop whatsoever. I had ther same thing happen to me 3 times this week at ambient temperatures of 0-7F. I have a 2001 Subaru Outback LLBean edition. I called a dealer in Massachusetts and the service gentleman told me the following. Either the cold brake fluid just needs to be moved around some in order to get the brakes working properly, or I may have gone through a puddle (causing ice buildup) or had a lot of brake dust build-up, and it was necessary to heat up the rotors to get good contact. Lately I have been stomping on the brakes a couple times right after starting the car, and I haven't had the problem (but it hasn't been quite as cold since I started doing this). The service guy did not recommend coming into the dealership for any modifications to the car. A similar thing happens with my wife's 2002 Outback (LL Bean Edition). First couple of times it occured, car ended up in neighbor across the street's driveway! Car will stop if really stand on the brakes. I'm beginning to think it's related to the vacuum boost for the power assist. Like the earlier comment, I found that really pushing on the brake pedal several times when the car is first started seems to free things up. Answer. I have a 2003 Outback LLBean, exact same problem.