GMC Troubleshooting Questions? Ask a Mechanic for Answers
Ok if it is a vacuum brake booster it will have a large metal vacuum booster behind the master cylinder,if it is a hydroboost system it will have power steering hoses going to the hydroboost unit behind the master cylinder. Pump the brake pedal several times and hold your foot on the brake and start the engine, if the booster is ok it should make the brake pedal drop while holding down on it
And the light stays on all the time?