Brake lights don't work , relaced bulbs, and circuit board, all lights work including third brake light but notstop lights

SOURCE: 95 k1500 tail lamps dont work. turn signal and

test the wires to see if power reaches the tail light, connect a test light to the wire and the chassis, if there is power present on the wire for the tail light the problem is the bulb or the tail light unit, but if there isnt any power you have to backtrace the wire to locate the problem, probably a broken or loose wire

SOURCE: rear brake lights wont work

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.

SOURCE: Brake lights don't work but third brake light does

I believe the flashers are on a different circute. It sounds like a bad ground.....or one of your taillight sockets is bad. I have a cavilier that had the same problem. I went to a junk yard and found some light socket s and swopped them. So far it has been working. The third light has its own ground. tail light sockets need to have a good ground, some times the ground from batt. to block, ground frome block to frame can be bad.
Think "ground"
Don't try and buy new sockets , unless they are just like the fasctory sockets. getting the entire tail light works good. Find the main tail light, the juce comes to it first, then passes to other side. Check that dide frst. work your wat over to other side, if u cant find a problem, work your way to the front and find out where the lights are grounded.
good luck
Doug