I have replaced the fuses and the pedal switch a couples times. I do not see a broken ground, but the brake lights will quit about a day after I get them going again. I am not an electrical type guy. Sorry if this has been covered a lot.
Joe

 

Look for a loose bare wire in the harness going back to the lights. A short will keep popping the fuse and it could be that the insulation is missing on a piece of wire or there is the remnant of a trailer tie in that swings around and randomly shorts.

 

Check the fuse block for corrosion. Mine had gotten some moisture on it, which in turn rusted and had poor contact. Next time they go out test both sides of the fuse with a test light.

 

The stop (brake) light switch located on the pedal is adjustable.

Pic (at lower left): 13480 switch threads into 13A629 retainer, which then snaps into the bracket on the pedal.

The retainer has probably broken, so it's no longer firmly attached to the brake pedal. This is a common problem on these trucks.

C8TZ-13A629-A .. Retainer-Stop Lamp Switch / Obsolete

1968/77 F100/350 / 1978 F100/350 & Bronco before serial number CC0,001.

MORLAN-SHELL FORD in West Plains MO has 4 = 417-256-3193.

WESLEY OBSOLETE PARTS in Liberty KY has 4 = 606=787-5293.

GREEN SALES CO. in Cincinnati OH has 74 = 800-543-4959.

 

Blown fuses indicates there is high current (amperage) on that circuit. There are either two wires coming in contact with each other or, there is a wire coming in contact with the frame/chassis.

 

Thanks for the help and ideas guys. It was a bad connection at the hazard flasher, the one with the white/red wire.