Whenever I turn the ignition on in my 1981 F-150 1/2 ton, longbed, 300ci 6 cyl., manual (4 spd.) tranny pickup, the red "Brake" warning light on the dash comes on and stays on.

Because the truck predates ABS, as far as I know this isn't related to low brake fluid. Still, I just verified that the brake fluid isn't low in the master cylinder. The brakes work fine and the parking brake works fine, it's just that the red dash "Brake" light stays on regardless of whether or not the parking brake is on.

I looked for the parking brake switch under the dash and at the top of the parking brake pedal arm, but I can't see it anywhere. I'm pretty sure it's broken and is what's causing the light to come on. My Chilton doesn't cover the switch, can't find it in any diagram in that book. Heck, I can't even find a picture of the switch by Googling!

Can anybody tell me where that switch is, how to see it and does anybody have a diagram of where it is or a picture of it?

Thanks in advance for any help you can give me.

 

iirc the parkingbrake light did not show up until 83. I know my 80 doesn't have a switch on the parking brake. there should be a sensor under themaster cyl. under the hood with2 wires on it. that is the pressuresensor for the brakes. unhook that and see if the light goes off. if it does, hook it back up and mash the brakes with the truck off and then see if it goes out.

 

A friend and I recently drained and flushed the brake lines, then filled and bled them. Just noticed this in the Chilton manual on page 354:

CENTRALIZING THE PRESSURE DIFFERENTIAL VALVE

After any repair or bleeding of the primary (front brake) or secondary (rear brake) system, the dual-brake system warning light will usually remain illuminated due to the pressure differ*ential valve remaining in the off-center posi*tion.
To centralize the pressure differential valve and turn off the warning light after the systems have been bled, follow the procedure below.

1. Turn the ignition switch to the ACC or ON position.

2. Check the fluid level in the master cyl*inder reservoirs and fill them to within 1/4 in.
of the top with brake fluid as necessary.

3. Depress the brake pedal and the piston should center itself causing the brake warning light to go out.

What they don't say is whether or not to leave the cap off the master cylinder while depressing the brake pedal and how slowly to press it.

 

Okay, so I tried the steps listed for centralizing the pressure differential valve. The light stayed on. Guess the parking light switch is broken, so I still need to find out where that switch is, how to see it and does anybody have a diagram of where it is or a picture of it?

Again, TIA for any help somebody can provide.

 

Thanks, I'll give that a try!

 

If you want to double check and make sure it's not the proportioning valve, look at the valve, there will be a wire going to it. Unplug this wire and see if the light goes out.

 

My proportioning valve went bad and the piston stuck off-center, which kept the light on. So I had to replace it. But the fact that you'd just bled the brakes makes me think it is just a balance problem, meaning that you have to STAND on the brakes, while sitting still, to center the piston.

 

IIRC it was in 1984 when Ford started hooking up that red light to the parking brake pedal; prior to that, it warned only of troubles in the master cylinder.

 

I have to add, it's been several months since my friend and I did the brake fluid clean, flush and replacement. Strange that it took this long for the light to come on.

 

If it has been that long then you need to ensure brakes are working on both ends. Something may be wrong, and that's what the light is for.

 

Thanks for that. Makes a lot of sense. However, I don't put that many miles on the truck, it's only been about 600 miles since we did the work.

This morning, I went outside and with the igntion on, did as that same friend advised: STOMP on the brakes with the truck not running and the lid closed and clamped on the master cylinder. After about two or three stomps, the light went out so just to be sure, I STOMPED the brake again about five times. It stayed out. I hope it continues to stay out.

My friend is quite a mechanic, swears that the only way to center the proportioning valve is to stomp the brake pedal.

 

I am glad you got it fixed.

 

Actually, that didn't turn out to be the problem. It came back. What fixed it was that my friend and I bled the master cylinder and then re-bled the brake lines. Turns out there was still a little air in the master cylinder.