One problem related to brake master cylinder has been reported for the 2005 GMC Envoy. The most recently reported issues are listed below.
In August 2005 on a trip, exiting off ramp brake pedal went all the way to the floor, was able to "pump" it and get response. After shutting vehicle off, setting & restart brake pedal again goes all the way to floor. Upon return to dealership master cylinder was replaced. A few months later brake pedal becomes extremely "spongy" with no stopping power. Dealership bled brakes and said problem was air in system. Still no improvement, extremely soft pedal and unit will not panic stop. Took back to dealership end of June 2006 and they replaced master cylinder a second time. Unit is still no better and in my opinion unsafe to drive if it will not panic stop. Dealership says gm has told them to order more parts, but do not know what the problem is.