Dash Brake Light

[67step100]

I noticed today when going out to my truck in the barn that the brake light was illuminated on the dash (to the right of the steering wheel). Only noticed it because I hadn't yet turned on the lights. My manual ('67 D100) says that the light will stay on if there is a drop in the hydraulic pressure system for the brakes. I checked the brake fluid reservoir and it is full and I do not see any leaks anywhere in the brake lines. Could this be a short? Or is there some other cause for the light to stay on? The light stays on even with the engine off and the key out of the ignition. Thanks for any guidance.

[712edf]

Something is shorted to ground I suspect. The brake light switch (at the pedal) itself does have power available even with the key OFF. But I don't think that dash light should work with key OFF. It comes on either when the valve inside the combination/proportioning valve is off center (due to a leak) or when the parking brake lever is not completely disengaged. But usually only when the key is ON/Run.

Any other dash lights illuminated? I think this is an issue within your instrument cluster wiring & not necessarily a brake problem itself.

Bucky

[PwrWgnDrvr]

The valve Bucky refers to is not a proportioning valve. It is found on the frame, near the steering gear. Its a safety valve only and its purpose is to sense fluid pressure differential between front brake and rear brakes caused by a leak, BUT ONLY WHEN there is a dual system master cyl. If you have a dual system, look on the inside of the tires for fluid leaking out of a wheel cylinder and running down while parked. But, if there is a leak, the master will show the loss. Earlier single pot trucks have a pressure sensing switch, usually screwed into a port on the master. If the brakes work fine and there is no fluid loss, its an electrical problem. Start with a new switch.

[67step100]

The valve Bucky refers to is not a proportioning valve. It is found on the frame, near the steering gear. Its a safety valve only and its purpose is to sense fluid pressure differential between front brake and rear brakes caused by a leak, BUT ONLY WHEN there is a dual system master cyl. If you have a dual system, look on the inside of the tires for fluid leaking out of a wheel cylinder and running down while parked. But, if there is a leak, the master will show the loss. Earlier single pot trucks have a pressure sensing switch, usually screwed into a port on the master. If the brakes work fine and there is no fluid loss, its an electrical problem. Start with a new switch.

Thanks Bucky and PwrWgnDrvr. My truck has the single pot fluid reservoir, not dual. There are zero leaks. I checked again at each of the brake line connections front to back and everything is dry. No fluid loss and the brakes still work. It feels the same as it did before the light came on.

When you say start with a new switch, are you referring to the ignition switch? Thanks.

[PwrWgnDrvr]

Start with a new pressure sensing switch. They look kinda like an oil pressure sender. It will thread into a port on the master, or possibly elsewhere on the brake line. It will have 1-2 wires connected to it.

[67step100]

Start with a new pressure sensing switch. They look kinda like an oil pressure sender. It will thread into a port on the master, or possibly elsewhere on the brake line. It will have 1-2 wires connected to it.

This is the only thing that has a wire going to it associated with the brake lines. It's a single blue wire. There are no wires going to the master cylinder. I've looked at the various suppliers and no one carries anything that looks like this. Any ideas? Thanks.

I also just realized that it must be a dual master since I have disc brakes on the front and drums on the rears, or is it possible to work both off a single?

[PwrWgnDrvr]

OK, that's it. But that is for dual systems. How many lines come out of the master? When you remove the cover, are there 2 compartments for fluid?
That valve in your pic has a piston inside. If the pressure in front and rear brakes is the same, the piston stays centered. If pressure drops on one side, the piston moves off center, closing the circuit and the light comes on. It is a safety sensor switch to tell you there is a problem somewhere.
Looking at your pic, it is obvious that the truck has major mods. Now you tell us! Normally that valve is mounted inside the frame rails with the wire coming off the bottom. Also, one port would not be plugged. Not knowing anything else about the system mods, its impossible to determine if there is a problem, or merely a design flaw during modification.
Simple answer to get the light to go off: pull the wire off the valve or off the light or remove the bulb.

[67step100]

OK, that's it. But that is for dual systems. How many lines come out of the master? When you remove the cover, are there 2 compartments for fluid?
That valve in your pic has a piston inside. If the pressure in front and rear brakes is the same, the piston stays centered. If pressure drops on one side, the piston moves off center, closing the circuit and the light comes on. It is a safety sensor switch to tell you there is a problem somewhere.
Looking at your pic, it is obvious that the truck has major mods. Now you tell us! Normally that valve is mounted inside the frame rails with the wire coming off the bottom. Also, one port would not be plugged. Not knowing anything else about the system mods, its impossible to determine if there is a problem, or merely a design flaw during modification.
Simple answer to get the light to go off: pull the wire off the valve or off the light or remove the bulb.

Here is a pic of my master and the blue wire going to the valve (black arrow). I initially thought it was a single because both brake lines looked like they were coming from the same compartment, but there are 2 compartments, the front being larger than the rear as you can see from the cover. I've posted the valve again showing which lines go where. Where would the plugged port normally have gone? Two to the rear? I unplugged the blue wire from the firewall and the light went out so it is no doubt the valve. Where can I get a replacement? Thanks again for the assistance and my apologies for getting things mixed up at the start.

[PwrWgnDrvr]

There is no replacement available for the valve. Yes, that is the std dual master. What the valve is telling you is that the pressure is not equal in both systems. Since there is no leak, the problem is likely the master. When the seals in the master wear, it is possible that one is more worn than the other and pressure doesn't build equally. The light is a warning that something is wrong. Don't ever assume its just a bad warning light. The FSM has all the detailed procedures to diagnose what the problem is.

[nutz]

just unplug it if light goes out its in the valve
bad master won't normally set that valve off because it needs a drop in pressure on one side to shift the valve over and you will have a low peddle
now with that being said if you have disk brakes and no proportioning valve it is possible to trigger that valve because of the different volume of
a caliper compared to a wheel cylinder
now if you unplug it and light is still one could be a short in the wiring

[martincom]

I believe you have another issue. That indicator should go off when the ignition key is in the off position.

Also, it appears you have a disc brake master cylinder (front reservoir is larger than the rear). Has your truck been converted to front disc brakes?

[PwrWgnDrvr]

I believe you have another issue. That indicator should go off when the ignition key is in the off position.

Also, it appears you have a disc brake master cylinder (front reservoir is larger than the rear). Has your truck been converted to front disc brakes?

From his post above:

....I also just realized that it must be a dual master since I have disc brakes on the front and drums on the rears,....

He could have all kinds of issues.....

[67step100]

Yes, disc brakes on the front. This is a recent issue. The light just came on and as I mentioned it stays on all the time, even with ignition off and key out. It does turn off if I unplug the wire coming from the valve at the firewall connector. That is why I am assuming it is the valve. The brakes continue to work and I do not feel any difference now than I did before the light came on.

What I can't understand is why the light comes on when there is no brake pressure? If its purpose is to show a difference in pressure between the two sides of the valve, there should be no pressure at all until I apply the brakes. That is, no pressure on either side of valve (thus equal) so no light. So why is it always on? If these valves are no longer available, what are my options? I had assumed this was some type of proportioning valve since there are disks on the front and drums on the rear. Or does the master cylinder do the proportioning?

[martincom]

If you disconnect the wire at the valve and the indicator goes out, then the valve is active.

On original Chrysler valves, the valves had springs to self center them and automatically "reset". I can't tell from the photo if this valve is original or not. Typically, the mounting type and fasteners would be an indication.

You are correct, some place in the rear brake hydraulic circuit there should be a proportioning valve. Sometimes, the proportioning valve is incorporated into the tee, that would be mounted on the rear axle. Sometimes it is inline, some where in the line feeding the rear brakes. It can also be incorporated into the pressure differential valve/switch. I'm going to speculate it is the latter, if there is an indication by the mounting that it is not OEM pressure differential valve/switch.

Some of these pressure differential valve/switches are not self resetting. You actually have to remove a plug and slide the piston back to center position, so the brake indicator lamp goes out. Ford had valves of this style. This might be what you have.

If you remove that plug to check piston position, wrap the frame rail and anything else brake fluid may drip on with aluminum foil. Brake fluid is a great paint stripper.

The master cylinder could be failing or low on fluid. A failing master cylinder would result in a difference of fluid pressure from front to rear, without any fluid leakage. However, usually you would "feel" the difference in the pedal as well as braking force.

[67step100]

If you disconnect the wire at the valve and the indicator goes out, then the valve is active.

On original Chrysler valves, the valves had springs to self center them and automatically "reset". I can't tell from the photo if this valve is original or not. Typically, the mounting type and fasteners would be an indication.

You are correct, some place in the rear brake hydraulic circuit there should be a proportioning valve. Sometimes, the proportioning valve is incorporated into the tee, that would be mounted on the rear axle. Sometimes it is inline, some where in the line feeding the rear brakes. It can also be incorporated into the pressure differential valve/switch. I'm going to speculate it is the latter, if there is an indication by the mounting that it is not OEM pressure differential valve/switch.

Some of these pressure differential valve/switches are not self resetting. You actually have to remove a plug and slide the piston back to center position, so the brake indicator lamp goes out. Ford had valves of this style. This might be what you have.

If you remove that plug to check piston position, wrap the frame rail and anything else brake fluid may drip on with aluminum foil. Brake fluid is a great paint stripper.

The master cylinder could be failing or low on fluid. A failing master cylinder would result in a difference of fluid pressure from front to rear, without any fluid leakage. However, usually you would "feel" the difference in the pedal as well as braking force.

Thanks. Yes, there does appear to be a proportioning valve incorporated into the tee mounted on the rear axle. The master cylinder is full and I don't feel any difference in braking force from before. I will remove the plug and see about resetting the piston position. I did locate this which appears to be exactly the same valve I have installed.

[712edf]

That combination or safety valve looks to be configured same as a stock one. Two lines coming in, two lines going out to front, one line coming out going to the rear. Where the plug is in the front sometimes has a pin sticking out that moves outward when the brakes are applied.

If you are getting a BRAKE (warning) light with the Key OFF & disconnecting the wire turns the light off, that tells us TWO things. The wire is providing ground to the light due to the safety valve being off center or defective, AND your instrument cluster is still getting power when the key is off, which it should not.

Bucky

[nutz]

would try disconnecting your alt
possible bad diode

[67step100]

712edf, good point and I had been thinking about that. Light should not come on until I turn the ignition key so I need to check the ignition switch. I will also try disconnecting the alt as suggested by nutz.

I would rather check off the easier fixes than start with the brake valve and going down that rabbit hole. I really hate working with brake fluid.

[PwrWgnDrvr]

Yes, but....who knows what the person did to the wiring for the safety switch when they did the disk brake and safety switch mod. Don't condemn the ignition switch until you determine where the +12 to the dash brake light is coming from and verify if it is correctly connected to a switched source.

[67step100]

Yes, but....who knows what the person did to the wiring for the safety switch when they did the disk brake and safety switch mod. Don't condemn the ignition switch until you determine where the +12 to the dash brake light is coming from and verify if it is correctly connected to a switched source.

Thanks PwrWgnDrvr. This however, is a new issue. The light has just come on and the disk brake conversion was done over 10 years ago so I would assume if it had something to do with that then it would have manifested itself at that time. I figure start with the simplest things first.