Brake Master Cylinder problems of the 2008 GMC Envoy

While on our way back to our motor home the brake pedal on my 2008 GMC Envoy went to the floor when I tried to stop in heavy traffic on hwy 503 in the state of washington. After pumping the brake quickly I was able to regain some braking power. When I called the local GMC dealer (carr GMC) in vancouver the service adviser confirmed that it was the master cylinder. Now this happened on Tuesday July 20, 2010, the service adviser gave me an appointment for Thursday July 22 between 9:00 to 9:30 am. I arrived at 9:10 am and was told that it would take 2-1/2 to 3 hours to diagnose the problem. Now this was the same person that told me on the phone two day prior that I had a bad master cylinder. Now I just drove this car 48 miles with the brakes not working properly. The dealer did not have a master cylinder in stock and was having one overnighted in from lancing, mi and would call me when it was repaired. Now I don't know of how many other vehicles this has happened too, but having a master cylinder fail on a car that has only 31,000 miles on it is not good and a real safety issue.