It appears that the Ecoboost 150's use a electric vacuum pump mounted behind the left headlight. Possible that it failed, or is having an intermittent electrical connection issue. I would have the dealer take a look at it and see what they can find. Check out page 3 of this attachment which details that these have that electric vacuum pump for the brake booster. Your dealer should already know this, if not run!! No I do not work for Ford, I work at a Deutz diesel, and Volvo penta distributer as a tech for a living. Hope this helps you get to the bottom of your issue.