The check brake system warning occurred shortly after purchasing the vehicle and numerous times intermittently thereafter. I became startled by the alarms the first time that it occurred while cruising at highway speed and applied the brakes. The brake pedal felt hard and without vacuum assist so I pressed harder to slow the vehicle. Suddenly, the vacuum boost recovered which in turn caused the anti-locks to engage with nearly a full scale lockup.

This incident was reported to the dealer who sold it to me and again to another dealer some time later at the first oil change. Both technicians essentially dismissed the problem with a "bring it in if it happens again". After speaking with a Ford rep, I found it odd that data for this serious problem is showing for every occurrence on the computer at his tier level. Meanwhile, the Ford Pass App fails to indicate the flaw while it does inform of things like an occasional pre-collision assist not being available (due to snow build up on the sensor) and security alerts (due to out of sequence vehicle egress). The app's vehicle health reports (Brakes & Suspensions) also have no indications of the problem.

Some online forums suggest a defective vacuum sensor. This sensor is readily accessible near the master cylinder, so I am wondering why this part wasn't replaced. If there is indeed a vacuum leak, it would explain my reduced gas mileage and weird shift points.

Update from Apr 13, 2021: Vacuum boost system check valve assembly was replaced. Seems to have corrected the problem.