One problem related to brake master cylinder has been reported for the 1999 GMC Sonoma. The most recently reported issues are listed below.
Rear brake rotors disintegrated after 22,000 miles. Had to be turned and pads replaced. Front rotors replaced at 32,000 miles same condition. Master cylinder failed and needed to be replaced at 32,800 miles. Pedal went to the floor and nearly went in front of a semi. Pedal can be pumped up but gradually goes to floor.