There is another thread going involving break lights on back of truck staying on, but my problem is that the light on the dash is staying on all the time (when truck is running).

Where should I start the troubleshooting for this?

F250 1979 Ranger Lariat 4X4
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It could be a bad proportioning valve, a mis-adjusted brake switch (the one touching the brake pedal arm), a front or rear brake issue, a master cylinder issue, just to name a few. I would go over each thing individually and check for any wear or damage, replace, repair, or adjust accordingly. You can get a proportioning valve at the junk yard. I would get a guarantee that it will work, whether or not it fixes the issue. Good luck.

 

I was going to start with the switch because they are cheap, but I wasn't sure that would be the problem since the rest of the brake system, including lights, seems to work fine. Is it possible to still be the switch considering all this?

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I don't think your brake light switch, located on the pedal, will have anything to do with the brake light on your dash staying on. The other thread is covering this as well, it got sidetracked momentarily. I'm no expert but it either indicates a pressure loss somewhere or the switch on the proportioning valve is not centered/or bad. FWIW, Chiltons has a procedure for centralizing the pin in the valve after bleeding:

1. turn ign to ACC or ON
2. check fluid in the MC and fill to 1/4 inch from the top.
3. depress brake pedal and the pin should recenter itself
4. turn switch off
5. check operation of brakes be sure firm pedal is obtained.

 

Another area to go bad is in the sensor wiring in the harness. To check it you can start by jumping across the studs at the pedal switch. if the lights go off there you go. Next would be to check continuity in the wiring outside the cab on the DS inner fender. The wiring harness comes out in a group and separates into the front light harness and the set going down to the proportioning valve. You can also check the proportioning valve switch. ordinarily it will be one of the three. Until I fix my breaks I just removed the bulb!!!

 

This light goes on when the proportioning valve senses a pressure differential between the front and rear brake circuits. The switch inside the valve can also short out, and keep the light on permanently. It has nothing to do with the stop lamp switch.

Try recentering the valve as Bama described. If the brake pedal is firm and you're confident there are no issues, then it's likely a shorted proportioning valve switch.

 

Is there not a switch in the E-brake/Parking brake mechanism to trigger the light when the parking brake is engaged?

 

Sedans yes, but not in this era of trucks.

 

You can unplug the electrical connector from the proportioning valve, the dash lamp should then go off.

(If it doesn't, there is a short to ground somewhere in that wire.)

Ground the terminal in the connector to the body or engine, the light should go on.

In the case of a failing master cylinder, there will be a pressure differential between the front & rear circuits; the pin (connected to the wiring) will get pushed to the low-pressure side, where it grounds out and lights the lamp.

Those proportioning valves can be disassembled & cleaned, too.

https://www.ford-trucks.com/forums/1...lve-issue.html