Only three things will make that one turn on. The fluid level is low in the reservoir, the parking brake is not fully releasing, or there is a leak in one of the two hydraulic circuits.
GM has a really weird master cylinder design. If there is a leak in the hydraulic system, a valve trips in the master cylinder to block the port to that front wheel and the opposite rear wheel. The brake pedal will feel normal, there will likely be no brake pull, and the fluid level will not go down, yet you are supposed to know something is wrong.
If the parking brake is not fully releasing, typically due to a sluggish cable, you might be able to observe a clue by watching the warning light. With the engine off and ignition switch on, watch the brightness of the light very closely while you apply the parking brake. If you see the very slightest change in brightness when you apply the brake, that switch is working properly and is not the reason the light is on.
If the fluid level is low, add a little clean new fluid but do not totally fill the reservoir. If the light goes out, the fluid was low either because of a leak or the front disc brake pads are worn and need to be replaced. Doing so will require re-seating the caliper pistons to make room for the new, thicker pads and that will push the fluid back up into the reservoir. That is why you should not totally fill it.
If the fluid is not excessively low, unplug the level switch in the reservoir. If the light goes out, the float for the switch may have sunk.
Finally, unplug the wire from the combination valve below the master cylinder if there is one. The pressure-differential valve part of that assembly is in the master cylinder but I cannot remember where the switch is. If that makes the light go out, look for a leak in the hydraulic system.
Tuesday, December 14th, 2010 AT 5:23 PM