No brake lights, everything else works: Turn signals/parking/4-way flasher. Have 12v inside the socket, ground to the captivity says good. I used that ground and with a jumper wire and go to the frame. Still nothing, I am stumped

 

Brake pedal switch? Did you have the 12V when depressing the brake pedal?

 

Yes I have 12v on the yellow/black(left) 12v on the green(right), this inside the socket.

 

So if you have 12 volts at the socket then what is wrong?
Are you getting 12v on the running light side of the socket?

So forget about the socket for now you need to check at the brake light switch because if you dont have power there then you will not get it out back.
So 1 of the wires to the switch will have power all the time. The other side will only have power when the pedal is pushed.
If no power to switch pull all the fuses and check with a meter.
If you have power in but not out then the switch is bad and needs to be replaced.

Now if you have power in & out of the brake light switch that power out wire goes up to the turn signal switch.
This switch cancels the brake light to the side the turn signal turned on to.
So the next thing to check is the turn signal switch.
The turn signal switch could be bad but needs to be checked or replaced.
Dave ----

 

On the switch side The green wire tones out to both y/bk and gr. Odd only one filament lights up :turn/park and 4way flasher

 

You have power in the bulb sockets, but the bulbs don't light up for brakes ... but do for signal lights? If was a bad turn signal switch, don't see how you'ld have power at the bulb sockets when the brake pedal is depressed. If those two brake/turn bulbs light up with signals, then they about have to with brakes. Are you having a helper mash the brakes while you are checking power at sockets?

 

I have a wedge to hold the pedal down far enough so that the plunger is out all the way

 

My truck is Trail Special. After looking under neath, there is at least 4 4-pin square type connectors. Those connectors and wires need to be replaced, they are somewhat crack and corroded. Looks like I need some colored 18 gauge wire and weather proof connectors. Oh boy let the fun begin

 

I saw where you said you had power in the sockets ... so the 4 pin connectors under the truck have to be OK, else you wouldn't have power in the bulb sockets where the bulbs are. If the signals work back there as stated, then the wiring including the 4 pin connectors under neath are all fine.

If it boils down to you just do not have brake lights lighting, but do have signals working, you need to look at the turn signal switch. You cannot assume that because the signals work that the sockets are getting power when the brake is depressed because brake light and signal light functions go through the turn signal switch.

There is no way that you are getting power at the sockets with the brake on but no lit bulb ... the same bulb that lights up with signal lights.

 

What bulb filament do the brake lights work on? My understanding its the longer/bigger one of the two, Turn signal/parking and 4-way flasher are flashing on the shorter/smaller one of the two.

 

Brake light and turn signal and 4-way flasher functions use the high wattage, obviously thicker filament, the tail light function alone uses the thinner low wattage filament. Anything else, you need new sockets to correct shorts inside. If you have 4-ways or turn signal doing anything with the thinner filaments, you have a ground failure at the back of the truck. Could be sockets or just where the ground wire attaches to the frame/body. What is happening then is the high wattage current is finding ground back through the tail light lower wattage side and that filament will get hot and light, but it limits current through the thicker filaments so much that they don't even get hot, much less light up.You have a ground if they light at all, you just don't have a good enough ground to support lighting the higher wattage filaments.

I replaced my rear light sockets years ago for a very similar reason. I had the same or very similar issue with a front above grille "park light / signal light" and new sockets cured that too. I once thought the turn signal switch controled grounds too, but I don't see how as my '77 has no dedicated ground wire going to the rear. 4 wires go all the way to the back after the fuel sender wire turns off at the front of the gas tank. Left brake-turn, right brake-turn , tail and tag lamps, backup light positives.

The "tail light / marker light /instrument illumination light circuit" is grounded or shorted in the multiple paths to ground provided by many other bulbs through their filaments to grounds.

I got ahead of myself ealier, was corrected, and totally understand my goof ... I just forgot something I maybe once knew clearly. My BAD, this is right.

 

Can't figure out why drivers side brake light comes on the dimmer filament. and not the brighter one.

 

Because by a defective ground, likely the socket ... the two filaments only see a ground path back through the tail light circuit with all the bulbs that includes, each one of those bulbs provides a ground path though it. It is of very low amperage carrying ability, limited by the filament which is a resistor in fact.. The current suddenly supplied by that brake light wire crosses the heavy filament to where it is sharing a common ground terminal with the thinner tail filament, but because the ground isn't really ground because of socket defect, the only ground available is the light amperage path through other bulbs in the tail light circuit, You take a 27 watt brake filament, wire it in series with a 8 watt tail filament, apply current to the circuit, you'll only light the 8 watt filament as it will not support the 27 watts needed by the thick one. The thick one just acts like a fat wire.

 

That socket has been change out. Odd thing is that the blinker and 4 way works with the brighter filament and parking is working on the dimmer filament . The grounds have been cleaned, the ground screw changed and the body sanded, Everything is nice and shining. Right side brake light works as it should.

 

When you put the turn signal on, the turn signal switch disconnects that brake light's brake lead so that the light waill flash. I think your issue lies in the switch in the steering collumn.