Two problems related to brake light on have been reported for the 1999 GMC Yukon. The most recently reported issues are listed below.
(rear brake line rupture) my wife was on her way to work when she applied the brakes the brake warning light came on. The pedal became real soft as if the vehicle wasn't going to stop. After returning home, upon investigation the brake master cylinder was almost empty. I found brake fluid coming out of a badly corroded brake line just behind the fuel tank. This could have been a very bad situation. We are the second owners on this vehicle, purchased in 2001. We have always kept it maintained and clean, including car washes after snow storms to remove road salt and sand. This should not be happening on an 11 year old vehicle, thankfully this didn't result in an accident. I have also noticed the fuel lines have corrosion at ever mounting location. So, I guess these are going to fail as well some time in the future.
I have a problem with my antilocking braking system. At low speeds there is a grinding noise and the braking is uneven; meaning it is not smooth, but jerky. This situation does not occur at higher speeds. After this situation the antilocking brake system light comes on and I do not have the problem.