Got a 1971 F100. Driver side brake light not working. Passenger side works. Tail lights work. Turn Signal works. Just brake light not working. HELP!

 

Check the bulb.
Check voltage at the bulb socket.. both terminals!
Clean and reseat the ground.
Inspect wiring.
Start measuring voltage back to the signal source.
Replace turn signal switch.

 

Welcome to the forum. You'll find that a lot of the electrical problems are grounding related. The ground point gets rusty, the screw rusts, and the terminal ring (on the end of the wire) rusts. Installing new(stainless steel) screws will help too. Adding more (and larger wire gauge) ground wires will never hurt either. That should cover it, HIO. I bet you've answered that same type of question so many times on this forum that you just cut and paste the answer.-lol- me too!

 

Keep in mind that you'll want to make a complete circle with your ground wires- by that I mean battery to engine, engine to passenger side frame, engine to drivers side frame, frame to the firewall, firewall to inner fenders. Frame to the starter will help too Here are some examples of the addition grounds that I've done:
In my case I could run 3 cables from the battery to the frame(next to the vin),eng & alt adjustment arm

 

A few more:
In this pic you can see the large than original starter and positive cables. On the starter solnoind the black (ground) wire comes from the firewall. I later added a jumper wire to the horn relay mounting bolt too. This is very important if you have newly painted fenders.

The pass side headlight was dim and the parking light did not work.-Both had bad grounds. So I extended the ground wire and put them on a SS stud. I added a new ground for the voltage reg too- one for the horn mount is next-when I replace the inner fenders and the rad support ( in march).

 

It's like typing " The quick brown fox jumps over the lazy dog"

 

If the turn signal works but not the brake light, you almost certainly need a turn signal switch. Sometimes you can clean and tighten them, usually not.

 

I think Gary is on the right track with a bad ground. Clean things up and see what happens.

 

Start with the simple stuff, but if the bulb is good and the grounds are good it could be a bad turn signal switch.

 

Welcome in, pal.
Swap the bulbs left to right, right to left. See if the problem follows. Go from there.

 

I agree with eliminating the simple stuff first but if the turn signal is working properly then the bulb and the ground are most assuredly good since the brake light uses the same element as the turn signal

 

 

If the turn signal on that side flashes but the brake light doesn't, it's got to be the T/S switch or wiring. Probably the wiring plugs for the lamps on the left frame rail about at the spare tire area. PO's loved to hack in to the wiring there to hook up wiring for a trailer.