electric brake booster

I think these use hydroboost brakes, with the electric backup for the main power steering pump. It sounds as though the pressure from the power steering pump is not making it to the brake system for some reason; if the power steering seems to be operating normally, I might guess (and it's purely a guess) that there's a problem with the valve system for the hydroboost brake accumulator...or perhaps a bad switch to turn the auxiliary pump on and off.

At first glance it would not appear to be related to the five star tune problem, but I don't totally discount that possibility. Vehicle systems are not always obvious in their interactions and failures!