2002 Outback Brakes - not working when cold

My wife's 2002 Outback's (LL Bean) brakes sometimes don't work when the car is started, either first thing in the morning when leaving for work or, less frequently, in the afternoon after a cold soak on the parking lot. First time it happened, ended up the neighbor across the street's driveway. Symptom is like the power assist has failed (no vacuum boost?). Car can be stopped by really standing on the brake pedal. Problem only shows up when outside temperature is below approx. 25 deg F. After 1 or 2 minutes of either idling or slow driving, problem clears up and car is fine for rest of drive. Also, through experimentation (it's been below freezing all week), found that pumping the brakes really hard several times when car is first started seems to get things working. Anyone else have similar probelms?