Brake Master Cylinder problems of the 2008 GMC Yukon Hybrid

Loss of brake pressure, pedal went almost to floor. Brake warning annunciator stabilitrak caution annunciator service brakes soon message on dic stabilitrak failure message on dic driving home from work, radio on, air conditioning on, driving for about 10-15 minutes a guy in front of me does a quick lane change and turn in front of me. I have to step on the brake hard but not super hard. When I did this the brake pedal went further than normal and all of the above warning lights illuminated on the instrument panel. A). I slowly pull into a parking lot (2-3 mph) and the pedal goes almost to the floor. Almost cannot stop. B). I shut off the vehicle and restart but the lights come on as soon as I press on the brake. I do this several more time, check brake fluid reservoir but the same results. C). After about the 6 time restarting, I hear a small motor running ( brake accumulator / modulator ? ) and the pedal begins to get firm again and the warning annunciators extinguished. D). I continued driving home slowly with no further faults. Didn't think of checking the voltage until everything was good, at that point it was indicating 15vdc. This problem has occurred intermittently many times since then. Have taken to keeping one foot on emergency brake. The dealership replaced brake master cylinder but this did not fix problem. Dealer ordered the electric pump (brake modulator) and two weeks later dealership states gm cannot locate any parts to repair the vehicle. Part remains on order. Vehicle not operable without brakes.