Actually, this is a problem with turbocharged gas motors. When I owned a Saturn Sky Redline, there was a TSB for the 2008 car which removed the electric vacuum pump from the system (GM figured that it was unnecessary). There were situations where shortly after first start-up, there may not be enough vacuum. So when you're backing out and hitting the brakes, you find yourself having to REALLY push down since you have no power assist. They ended up adding the part back in 2009 and adding it back to 2008 cars that customers experienced the issue in. Typically the brake system is boosted by engine vacuum. With a turbocharged motor, you have times where the manifold goes positive pressure or doesn't produce enough vacuum because of mild boosting from the turbo. So many turbo cars get an assist from an electric or mechanical vacuum pump. Seems like the assisting vacuum pump failed on your ride and you were relying purely on the engine's vacuum which isn't consistent, so it's in those short inconsistencies where the assisting vacuum pump helps shore-up vacuum assist (along with a vacuum reservoir I believe). I'm surprised I haven't heard of this so much.
