Originally posted by DAS
I did NOT say I loose my power assist, I LOOSE MY BRAKES. NO STOP, PEDAL HARD AS A ROCK. I am 6'3" and 200 pounds, I can not stop the car with both feet on the brake. I have experienced the lack of power assist, this is alarming, but not unsafe.
Sometimes I hear the ABS cycling, other times I do not. When it is well below zero the brakes DO NOT WORK for about 30 seconds after start up.
When there is no power assist (the vacuum is all gone in your booster), you will have a rock hard pedal. I just experienced this for the first time recently and my experience sounds like yours. It is the check valve in the brake booster line. Moisture in the valve freezes and causes the valve to stick.
You'll probably have a regular pedal when you first step on the pedal to shift from park. This is using up some of the reserve vacuum in the booster (this is the function of the check valve - to keep a reserve of vacuum in the booster should the engine fail while you're in motion - the reserve of vacuum will be enough to have power brakes for one or two pumps of the pedal). In fact, when I pulled my hose to spray wd-40 down it, there was a hiss as the reserve vacuum was lost from the booster.
The second or third time you step on the pedal you will have no power assist and a rock hard pedal. I'm guessing that after a bit of warm air gets in the vacuum hose, the valve loosens (or maybe the force of mashing on the pedal causes it to break free or maybe the build up of vacuum between the engine and the check valve causes it to free up) and you have regular brakes again.
When I had the booster hose off, I blew into it from both ends, and can confirm that there is a check valve in there and mine was working, but it was warm at the time. I was tempted to throw it in the freezer for several hours and see if it would stick.
The next time it's that cold out you can figure out if it's the check valve. Start your car. Step on the brakes - pedal feels normal? Step on the brake again - pedal is rock hard - it's the check valve.