86 chevy 1500 4x4 brake lights wont work but running light do what could be the problem

check your fuse and relays

Did you check the park fuse, or the orange wire going into the headlight switch for power ?

I'm having the same problem on my 86. Brake lights won't work. All the running lights/turn signals/reverse lights work. I put in a new turn signal switch/harness and new brake light switch with no change. Still no brake lights. Any help would be highly appreciated

you have a bad ground to the taillights , the signal is looping back to the front light grounds

You need to own a VOM Meter and know how to use it.
You need basic electrical knowledge
I would suggest buying a factory wiring diagram,only if you can read one. Otherwise,just get an estimate, at a repair shop

the break light wire on the lumina runs through the blinker switch or multifunction switch on the colum.. check there as well as at the tail light sockets. on the late 90s gm cars the sockets had a tendancy to rot away... use a test light at the brake light wire in the trunk and have a friend apply the break. if there is power then may have to get new tail lights. or replacement bulbs.

Flasher module (under dash by left leg); possible fuse in panel driver side dash by door; possible relay in power center under hood on driver side by brake booster.

check or put a ground wire on the light. maybe its bad. run from brake light to a suitable ground, to see if it works.

same bulb but 2 seperate elements. wiring diagram u should be able to obtain from your local auto parts store like autozone(preferred)

It sounds like you should check for power at brake pedal switch. Hook up a test light to the switch and press the brake pedal. If the light does not go on and you have power in the wires to the switch then the switch should be replaced.
I hope this helps you out