How do you reset a break light - 1999 GMC Jimmy

They should reset themselves or "go off" by themselves. Its possible you have instead a short or broken wire that was giving you a false indication in the first place

insert key into key slot and press the start stop button once WITHOUT putting your foot on the break. You should see numerous lights turn on,on the dash
weight for any warning lights to go out, in some cases, like low fuel warning light, you my need to press the BC buttonon the end of the indaca tor stalk once to remove them
press and hold the black button, on the bottom left of the speedometor, but keep holding down the button untial the next picture appers then release the button.
to cycle threw one by one all of the picture optctionsfor each service press black button once. when you arive at the SERVICE that you wish to reset press AND HOLD THE BLACK BUTTON until the text RESET?is visible under the picture then let go,finily press and hold the button again to conferm the reset.

have the fault code read and replace any faulty sensors. Have the system reset

try to read and clear the fualt code with car diagnostic scanner

By bleeding the brakes you cause uneven pressures in each half of the dual hydraulic system. Therefore, the sensor switch that detects this pressure imbalance has operated and will remain set as indicated by warning light. The switch is either located on the side of the Brake Master Cylinder or on the proportioning valve on the firewall.
To reset detecting switch, first disconnect wiring plug (Light should go out with plug disconnected) and remove switch. You may hear a click as the pressure difference piston resets as you remove the sensing switch. Pump the brake pedal a couple of times to ensure piston has reset and then replace switch and wiring connector.

there is no reset. if the light is still on, there is a problem with one of the pad sensors or the connection. also, make sure that the front pads are not worn enough to trigger the light.

With a code scanner or I do beleive it will clear after 100 starts.

if you changed master yourself ,& bled out air must have computer hooked up for reset

Make sure you check the spare tire too; had this problem on another model and the dealer said it was the spare which is also monitored.  Good news in case you need it one day.

tire reset, light on dash will come on if brakes notice a pressure change in tires/breaking or if any maintence to tires, breaks, change + or neg in air pressure. to reset push and hold button for @3 sec.