1998 gmc sierra brake lights

Try wiggling the hazard light switch while pressing on the brakes.The taillights are connected to the turn signal switch before it goes to the brake switch.The contacts on the hazard switch are probably dirty(I remember this problem years ago,same symptoms-3rd light works but l/R tailights not working).You may just be able to turn the hazards on+off a bunch of times to clean the contacts instead of replacing the switch.

There is no adjustment. The park lights are on a different circuit than the brake lights.
Do you have a third brake light that works. Its also on a different circuit but works from the same switch. Do your turn signals work. They use the same filament as the brake lights.
Also, the brake light circuit goes through the multi function (turn signal) switch.

Since the brake lights and turn signal lights use the same filament in the bulbs, the brake light circuit will pass through the turn signal switch. When a turn signal is used it cancels out the brake light for that side.

Let me know if your third brake light and turn signals work in the rear.
Also, if youy have not already, check/change the bulbs. The type of sockets those lights are in also get burned so look at them well.

Usually when both lights fail it is due to a poor ground wire attached to the chassis of the vehicle. This is done to save wiring and would most likely explain why the other
lights still work. Hope this is useful.

Try this possibilities to locate the fault. First get your voltmeter or 12v test lamp out. Check the 30 amp Brake (#2) maxi fuse in the under hood junction box.Fuse should have 12v at both sides at all times.
If okay proceed.
Go to the brake light switch. The orange wire should have 12v at all times. The white wire should have 12v when stepping on the brake.
If okay, proceed.
Next thing is the brake lamp relay in the underhood fuse box. Swap the relay with one that looks just like it and recheck the brake light operation. If the relay is no good replace it.
If the relay is swapped and still not working, see if you can feel the relay clicking when someone else steps on the brake pedal.

batt is power from the maxi fuse 12v all times
sls is the white wire from the brake switch 12v with brake applied.
gnd is body ground 0v all times
mfs goes to the multi function switch 12v with relay activated
the circuit is simple—power from the brake light switch white wires turns on the relay which then sends power back to the multifunction switch for distribution to the rear lights.
if you jump terminals batt and mfs the brake lights should come on.
If you have no power at terminal sls with the brake applied then you have a wiring fault white wire between the brake light switch and the relay
Thanks. Keep updated for any more query. You can rate this solution and show your appreciation.

Sounds like you need to replace the drivers brake/tail light.

Check the switch on the brake pedal, connect the wires to each other and if the lights work then the switch is at fault, if they do not then check fuses, wiring etc.

brake lights and turn signals work off the same curcit, it could be the fuse or the brake light switch !!!!!!

have you replaced the bulbs?

check the ground cable for the rear lights its at the back on the frame