Brakes Failed problems of the 2008 Subaru Tribeca

When the outside temperature dips below 20 degrees f, the brakes fail to work. This has happened 5 times now in the year 2014. This has happened each time my car has sat out in the cold temperatures (<20 degrees) for 8 hours (while at work). When pulling out of my space in the outdoor parking garage, my brake pedal becomes hard and I am unable to depress it. My car will not stop. Applying hard pressure slows it down. This has happened on dry pavement. I once had to use the emergency brake to stop. It has also occurred once after I was operating the car for at least 15 minutes. I have had the car to the dealership twice in the last 3 months since this started. Each time they are unable to duplicate the problem and cannot find anything wrong. I have also emailed Subaru customer service and they stated the dealership would have to duplicate the brake problem in order to fix it. This is a huge safety concern; not to menion I have 2 small children traveling in the car with me. No one seems that worried about it but me. It is all over internet car forums that this happens with Subaru brakes when the temperatures dip. The dealership and Subaru claim they have not heard of this problem. I am extremely frustrated with Subaru.