Answer:
The CHEVY / GMC Medium duty trucks have power assisted hydraulic brakes. The assist comes from the power-steering pump (not vacuum as with cars or light trucks). Pretty sure the brake system trade name for the C4500 series truck is HYDROBOOST and the trade name for the C5500 is HYDROMAX.
This brake system incorporates monitors for various things that can go wrong. When it does the dashboard BRAKE light (to the left of the speedometer) will come on as well as the SERVICE BRAKE light which is to the right of the steering wheel below the radio - it has a picture of a wrench. At the same time the warning CHIME will sound continuously. It will not silence until you fix the problem.
On my 2003 C5500 truck I discovered after much aggravation that the warning chime and light was sounding because of a failure in the electric backup to the HYDROMAX. The HYDROMAX system has an electric pump to power the HYDROMAX in the event the engine stalls while driving. Without it you would loose power braking if the engine stalled while driving. This electric backup system is checked every time the engine is started. If it does not come on, the warning light and chime will come on shortly. The pump is mounted at the firewall under the hood behind the steering wheel attached to the master cylinder. The pump is powered by a 40 amp plug in relay located right next to the electric motor.
On my truck this relay went bad and changing the relay solved the entire problem.
As a side note: if the brake fluid is low in the reservoir, the dash BRAKE light will come on but not the SERVICE BRAKE light.