[GM-power]

Have scanned the system for any trouble codes? GM has two bulletins that imop deal with your situation. One deals with checking the master pressure sensor(if it reads pressure while applie, check sensor, wiring etc). Second one deals with circuit 6311(it's a low reference circuit at 9v, this circuit run through all the modules, except the BCM, you'll need a meter to check your voltage at the ECM. For normal brake pedal operation it will be at 0v released and 12v applied. If you have 9v while it is released a open/high resistance in the circuit or a internal issue with the BCM may be present. I would start with scanning it, I kind of doubt the second bulletin I alluded to is the issue. I'm on my phone so I tried to write them out as best I could. Hopefully it's somewhat helpful! Lol