79f150 4x4 300 4spd.

We had the bed off, cab corner fix couple weeks ago. Put bed back on, tested lights. All seemed well.
Went out to dinner last night. Noticed behind me it wasn't glowing brighter red when brakes applied.
Figured, meh, the little brake switch again under the peddle came loose.. etc. I'll look tomorrow.
Go out today, found the fuse for brakes blown. Replaced. Tested - Just a VERY FAST blip of a brake light and nothing.
Turn on running lights, bam they work fine. Running and headlights, they work fine.
Also, noticed hazards don't work in any configuration.
Now, I 'think' we tested brake lights after bed was back on without running lights... but now I don't know.
-I have bypassed the brake switch under the peddle. - same thing.
-Fuse is still good after replaced.
-Volt meter shows a blip of 0.01v when testing on the fuse panel.
-Behind the fuse panel, things look clean.
- As a side note, my reverse lights don't work either. (replaced bulbs, bypassed reverse switch on tranny, fuse, etc.) This happened before the brake light issue.

I just don't get why they work fine with the driving lights on.... what should I look at? :/

 

So with the running lights on the running light are normal brightness and you hit the brake and they get brighter?
But with no running lights you get nothing right?
You did not post if you have 12 volts going into the brake switch and when you push on the pedal you have 12 volts out, so do you?
With out knowing that we cant go forward with testing.

Also when you removed the bed how did you go about disconnecting the tail light wires?
Dave ----

ps why is this in the 300 engine area and not say in your year truck area or the electrical area.
I think you would get more answers in the year of your truck area.

 

- Yes.
- True with the exception of a very fast 'blip/flash' of light'.
- Thanks, just tested. I do get 12v at switch.
- Removing the bed just unplugged the harness back there by the bumper and plugged it back in.

Ops. sorry, just a habit of this link I have.

 

So the 12v that is going in do you also have 12v going out when you push the pedal?
If you un-plug the harness again and test on the truck side what do you get?
Remember 1 wire will be left brake / turn, 1 wire right brake / turn, 1 wire running, 1 wire backup light. You may also have a ground wire in that harness?
Dave ----

 

Thanks man. hope I'm following along.
So here at the peddle switch is 12v.
Here at the rear harness under the bumper is a 0.01 (fluctuates at first .03, then drops to .01 sometimes to .00) when brake applied.
I just looked at all the wire harness going from front to back and don't see any weirdness..
(those cut wires are for license plate lights which I've never had)

 

Here at the fuse area, I am getting the same weird 0.01, 0.02 ish. with brake applied.