Two problems related to brake master cylinder have been reported for the 2006 GMC Sierra. The most recently reported issues are listed below.
Was coming to a stop light and brake pedal went all the way to floor. Had to step on several times to get stopped. The factory electrical wire in factory loom in factory clamp hooked to brake line wore a hole through brake line to right front wheel. Wore through directly under the master cylinder. Luckily I was not going at high speed or this could have been a fatal incident!!!!!!!!! this is something that come from factory and needs to be addressed.
2006 Sierra denali with just over 75,000 miles. With 1/2 block warning and only traveling at a speed of 20mph, I was unable to come to a complete stop. The brake pedal was all of the way to the floor when I ran into a van in the intersection. I felt very little resistance from the pedal and the abs definitely did not go off. There was no snow or ice on the road surface. However after towing the vehicle to the local dealership, I was told nothing was wrong with the truck. Two independent third parties have suggested this could be a master cylinder issue even though GMC gave it a clean bill of health.