02 Passat Power brake loss when cold?

I recently bought a 02 Passat wagon fwd 1.8T auto. The temp here(Mpls) lately has been in the teens. When we leave the car in our attached garage where the temp never get below 35 it is okay. When we leave it outside overnight the brakes have no power assist and is not drivable. If I pull it into the garage and leave it inside for a couple of hours it is fine. Any ideas? Dealer can't get it in for over a week. Could there be a check valve on the booster freezing up?
Mike