Brake Master Cylinder problems of the 2006 Nissan Altima

Brakes do not engage when car has been left in cold, usually below freezing. Brakes usually engage after I hit the pedal once, twice hard. Dealership stated master cylinder problem and replaced; now less than two weeks later same thing is occurring however not as cold and not as frequent as during the major cold spell in area. Dealership has it again overnight to see if they can replicate issues. Spent major bucks on master cylinder and am afraid to trust my car's brakes, plus perhaps having to spend more money on a different issue. Have seen others respond to this issue but find no major recalls; only a Nissan service campaign at some point to check brake pads and master cylinder for leakage. I have no idea if dealership ever checked my pads/master cylinder, been going to same location for almost 7 years.