Hey all, hoping for some help with a brake light issue. I have a 1980 F150, with manual transmission. The front has disk brakes and the rear has drums. The red warning light comes on in the dash when I put the key into ACC and also when the truck is running. I did put all new brake lines on this truck. What I've done so far to try and fix it:

1. Bled all four brakes with a pressure bleeder. I made sure to let the brake fluid run for 2-3 mins after I saw the last air bubbles to make sure all air was out.
2. Brake light was still on, so I opened up one of the outlets on the master cylinder and then had a helper depress the brake. Fluid sprayed out of the opened outlet. Brake light never went off.
3. Opened up the other outlet on the master cylinder and depressed the brake. Fluid sprayed out again but the light did not turn off.
4. Repeated the process, one outlet and then the other. Still the brake light is on.
5. Rebled all the brakes with the pressure bleeder.
6. Used the pressure bleeder while opening the outlets from the master cylinder, one at a time. Fluid leaked out but still warning light stayed on.
7. Pressed down hard on the brake pedal, with the truck in ACC and with the engine running. No luck. Did this with different speeds and pressures on the pedal.

I'm not sure what to do at this point. Apparently the plunger within the proportioning valve needs to be centered, but attempting to by opening up the outlets with pressure doesn't seem to fix it. Any tips?

 

Two different things can turn the brake light on, the proportioning valve or the switch on the parking brake. Go down to the proportioning valve, there should be a wiring plug with a single wire going to it. Unplug this wire. If the light goes out, you are correct the proportioning valve is your problem. If the light stays on, then it's the switch on the parking brake. Reach down and take your hand and lift up on the parking brake pedal and see if the light goes out.

 

Thanks Franklin! I forgot to mention, I saw your previous posts on the subject and did try what you suggested. The brake light DOES go out when I unplug the connector to the proportioning valve. The white connector that senses the plunger is new.

 

Looking more into this, I didn't bench bleed the master cylinder. Could this be the reason, even with using a power bleeder? And is there a way to do it on the truck or is it better to bleed it on the bench?

 

The power bleeding should have taken of that.

A little more history may help. Is this a daily driver? Derelict dragged from a field? New problem out of the blue? First time the truck has moved in years?

Does the valve have a protruding pin on one end? I wonder if the valve has seized from lack of use. Normally the valve innards that turn on the light stay put for years on end. It only moves when pressure is lost on one side. If the valve has the external pin, you might be able to work it free.

Does the valve have an external switch? On some models, I think the switch can be unscrewed and replaced separately. Maybe the valve itself is fine, but the switch is jammed.

This outfit carries replacements if needed:

https://www.carolinaclassictrucks.co...ion-valve.html

 

I just went through this on my 82. Could not get the proportioning valve to move after I bled the brakes and the light came on. I tried all of the things you did. I ended up taking the valve out of my parts truck ensuring that it was in the correct position (it did not cause the light to come on) before installing it. I installed it and used a pressure bleeder on the master cylinder. I had my brother help me open the right side front and back bleeder valves at the same time then go to the left side at the same time as to not change the pressures to allow the valve to move. We then closes each front and back valve at the same time. This fixed my problem and the light has stayed out. You could try taking it apart on the bench and freeing it up if you don't have a spare one like I did. I remember thinking they look like they come apart and that I should see why mine got stuck. Good Luck.

 

I know my 81's don't have a Ebrake light switch and he has confirmed it is the other valve / switch.

Karl this truck has been pulled apart and a frame swap.
I don't know what the deal was with the brakes before that but the truck was a rusty mess and why the frame swap.

The valve needs to be pushed to the center to turn the light off and the only way to do that is to remove pressure from the other side of the valve.
Not knowing what side it is pushed to you may have to try both sides.

Start at the rear and loosen the farther wheel bleeder and have someone push the pedal to the floor. This should push the valve over to the no pressure side now close the valve and see if the light went off.
If not try again and if still not off try doing it to the front.

I have heard some just get stuck from rust and junk in the fluid over time.
Dave ----

 

Hey, I actually learned the power bleeding from your how-to post! Not sure how much the previous owner drove it, but once I got it, I started doing a frame off resto and it hasn't been driven in four years now. As Dave pointed out, it was a rusty mess.

It does have a pressure release valve that looks like a pin coming out of the end... Do I try to loosen it up and get it to move in and out? It does have an external switch that I replaced (it screws in and is white in color).

Thanks JBruce... Good to know others had the issue. If I can't figure it out with a couple more tries, I'll either tear it apart like you said or buy a new one.

Thanks Dave. I don't think mine has the ebrake either. I'll try pushing the pedal with a front and then rear bleeder open and see what happens.

Now as pointed out above I see there are proportioning valves for about $60 that apparently work for 76-86. Does that sound right, that years 76-79 have the same spec as 80-86? I'll try what Fuzz suggested but if that doesn't work it may be better use of my time to just get an aftermarket one that'll work, since I seemed to have done everything I can besides take the valve apart.

 

One thing that is not standard but might be useful, I wired in an electric choke to what turned out to be a wire for the brake light in the cab. I believe it was purple and when I had it running the light would stay on no matter what I did. So if someone has fiddled with your wiring you may have a false positive.

 

If I was not clear it is the pressure from the master that pushes the valve over to center it.
The master PSI is a lot higher than the PSI you used for pressure bleeding the system.
Also if that dose not work it could be stuck and like you said it might be best to just replace it but I think I would try the bleeder & pushing the master to see if it will center just for S&G's

He knows it is the brake valve as he pulled the wire to it and it went out.
He just needs to center the valve for it to go out.
Dave ----

 

I had that problem the light was on the dash I checked the fluid level because the cap leaks need a new rubber I found the rear cylinder was low topped it off lights still on I applied the brakes really hard making a panic stop that did not skid the wheels but hit the brakes hard they must have shifted that piston and light went out for good so I would suggest try applying the brakes hard

 

On my 6.9l IDI the brake warning light will come on with a low vacuum to the booster. The 6.9 has a separate vacuum pump to provide the vacuum but gas trucks will have vacuum supplied from the intake side of the engine. If you are low on vacuum the light comes on for me. Usually requires replacement of the vacuum pump.

 

Gas does not have a low vacuum alarm / light like the diesels and being he pulled the wire off the valve / switch and the light went off it has to be that valve / switch.
Dave ----

 

I wasn't sure about gas. I'm not sure where the low vacuum sensor/switch is but it actuates the brake warning circuit for sure.

It makes sense that gas wouldn't need that because no vacuum means no go. Brakes would be down the checklist from there.

 

The low vacuum switch on my diesel is mounted on the pass side inner fender.