Brake light on dash coming on.

[Chvy2NV]

Truck is a 67 with 6 wheel disc in front. Drums out back. Took the truck for a drive the other day and the brake light on the dash came on and stayed on. Few hours later. I was leaving a friends house. And the light didn't come on as I left home.
Today. I took the truck for a drive and the darn thing came on again.

I don't think it matters. But fluid level is good. And E brake is off.

Not sure what that light is for ??
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[customcruiser72]

The light on the dash is a brake failure warning light and is probably an indication of a pressure loss in your brake lines. This can happen if a wheel cylinder fails or a brake line lets go or your master cylinder bypassing.
James
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[SS Tim]

The brake light is set up from the factory to warn of a pressure differential between the two brake circuits. Typically this is due to a leak or a master cylinder pressure problem. Now in converting your truck to disc brakes this feature should be in your proportioning valve as well. The switch in the prop. valve is grounded when the internal spool moves off center. If the light is staying on there is either a brake problem or an elecrical problem. The light should only come on during the start up "bulb check" unless a park brake switch has been added (a good idea). Driving around with the light "on" is not a good idea especially if you don't know why.

[Chvy2NV]

Thanks guys. Where should I begin to look
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[SS Tim]

First thing that comes to mind is the fact that it went out. This leans me towards a possible electrical issue as a starting point. The wire to the switch may be chaffed and grounding somewhere. It should be a tan wire. The switch itself might have an issue. Of course a quick look around for any sign of leakage or seepage anywhere in the brake system is also called for. Take a real close look at the rear drums on the backside if you can't pull the wheels and drums first.

Now your front brakes have been converted to discs up front with six lug rotors. During the conversion the factory distribution valve should have been changed to a proportioning valve. At this point posting a picture of the master cylinder and the valve would be a good idea for remote help.

[gvw5400]

X2 This is very sound advice from SS Tim. Brakes and steering are non negotiable items for you and your trucks well being! Diagnostics are always "fun", thankfully this site and the people on it are a HUGE asset in chasing the gremlins down. All the best to you in this process.

[jocko]

the oddity here is that your light is coming on after you drive it for a bit. If it were just a pressure differential (which is what is indicated if the switch is working correctly), then, once you have a pressure differential, it's going to remain until remedied (bleeding, refill, leak source repaired, etc). So, this is a bit odd. I'd do like Tim said and check for obvious leaks 1st, especially at the rear wheels. But the intermittent nature would lead me to lean more toward an electrical anomaly IF there are no obvious leaks (and that's backed up by full fluid levels). Keep in mind that a leaky rear brake cylinder might not be evident until you actually pull the drums. Also, the indicator on the dash is not related to the parking brake.

Also, some additional info would be appropriate -

Did this all start right after your front disk conversion or has it been fine for awhile and then got bad?

Did your conversion include a booster/master combo for disk/drum as Tim referred to? If not, you definitely need to get a new disk/drum master cyl and swap the distro block for an appropriate prop valve.

[SS Tim]

Guys the brakes were converted before he bought the truck. Went through the "build thread" last night and it is light on specifics.

[Chvy2NV]

Thanks Tim. I'll try and get a better pic up of the master cylinder. Sorry for not replying sooner. I hadn't noticed their were more replies. Have to change the settings. Yes the disk conversion was already done when I bought the truck and the light had never come in in the past. (I've had the truck for almost a year now). The light came on that first time and a few days ago and I looked around the master cylinder for any obvious leaks or anything abnormal. I unplugged the wire going to the pro portioning valve switch and the light turned off shortly after that. I drove her home that night with light off.
Then this past weekend. I drove it again and the light came on immediately.
Thanks for the help guys!!!
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[Chvy2NV]

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[Chvy2NV]

I had a chance to look under the truck today and did not see any leaks on all 4 corners.

Im starting to believe the issue is electrical rather than mechanical. I noticed when I started the truck for the first time today. The truck was cold and didn't start right up. After 2-3 attempts. It fired up and the BRAKE light came on and stayed on. Truck sat outside idling for a few minutes. I Shut it down. And about 15 minutes later I came back and started the truck again. Because it was already warm it fired right up and no BRAKE light.
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[Stocker]

This probably has nothing to do with your light problem..... but with front discs and rear drums, you should have a proportioning valve. Your pics look like a distribution block, which would have been correct when the truck had drums front & rear. Also, I believe the front brake lines should be smaller diameter with front discs... I'll check tomorrow to make sure. The prop. valve & smaller brake lines are b/c discs require greater hydraulic pressure than drums.

[cdowns]

doesn't look to b the right master cylinder either// r both compartments the same size? disc/drum should b different size for the fluid

[heathbirkendahl]

I'm having a similar issue. The brake light comes on when I push down hard on the pedal. So we replaced he master cylinder and proportioning valve. No leaks on any corner. I even pulled the rear drums and checked adjustment and for leaks. System is dry as can be. Light is still coming one.

Could this be related to any headlight switch and/or lighting? I recently did headlights, LED back up lights and new hi-beam switch.

Any help or guidance would be appreciated GREATLY!!!

Thank you,
Heath

[Stocker]

Not sure what might be causing the light to come on, but unless the wiring is hosed up, it shouldn't have anything to do with any other lighting. Lift the wire off the prop. valve and see if it still comes on.

[heathbirkendahl]

Originally Posted by Stocker

Not sure what might be causing the light to come on, but unless the wiring is hosed up, it shouldn't have anything to do with any other lighting. Lift the wire off the prop. valve and see if it still comes on.

Light does not come on when the wire is disconnected. Sounds like its time to start tracing break lines.

Thank you for the replies!

[Stocker]

Let's see if I get this right -- if there is an unbalance in pressure between the front & rear braking circuits, the prop valve senses it and grounds the brown wire, causing the light to come on. It's often an issue after brake work has been done, but usually stays on until you 'reset' it. If it stays on, it can usually be fixed by pressing hard -- very hard -- on the brake pedal. This will re-center the pin and the light will go out.

That, as I understand it, is how it's supposed to work. Yours sounds a little different, though. If I had to guess, I'd say there's an issue with the MC, but you have a new one. So.... is there a chance it was not properly bench-bled prior to installation?

[AussieinNC]

1/ Master cylinder pictured is drum/drum and most likely still has residual line pressure valve in place.
2/ Brake warning light switch has most likely had the end of the trip plunger broken off by excess brake pedal pressure "to reset the light".
3/ Brake bias block is drum / drum....needs to be changed out.

The switch has a mind of its own as it does not have the centering trip extension...disc calipers take nearly twice the fluid volume to operate against a drum wheel cylinder...

No booster ?

The fix is simple:

1/ Add a booster
2/ Add correct master cylinder and proportioning valve with new brake warning light switch...

WARNING...leave the switch out until after you have finished bleeding the system.

3/ You will need a few different brake pipes as the fittings are different sizes on true disc / drum systems.

If you have a big cammed engine, maybe you should look at a hydroboost system...

Have fun...

[heathbirkendahl]

Originally Posted by Stocker

Let's see if I get this right -- if there is an unbalance in pressure between the front & rear braking circuits, the prop valve senses it and grounds the brown wire, causing the light to come on. It's often an issue after brake work has been done, but usually stays on until you 'reset' it. If it stays on, it can usually be fixed by pressing hard -- very hard -- on the brake pedal. This will re-center the pin and the light will go out.

That, as I understand it, is how it's supposed to work. Yours sounds a little different, though. If I had to guess, I'd say there's an issue with the MC, but you have a new one. So.... is there a chance it was not properly bench-bled prior to installation?

Thank you Stocker for the reply.

We bench-bled if for sure. Truck stops great and does not leak. I'm leaning towards another bad proportioning valve.

[Steeveedee]

Originally Posted by AussieinNC

1/ Master cylinder pictured is drum/drum and most likely still has residual line pressure valve in place.
2/ Brake warning light switch has most likely had the end of the trip plunger broken off by excess brake pedal pressure "to reset the light".
3/ Brake bias block is drum / drum....needs to be changed out.

The switch has a mind of its own as it does not have the centering trip extension...disc calipers take nearly twice the fluid volume to operate against a drum wheel cylinder...

No booster ?

The fix is simple:

1/ Add a booster
2/ Add correct master cylinder and proportioning valve with new brake warning light switch...

WARNING...leave the switch out until after you have finished bleeding the system.

3/ You will need a few different brake pipes as the fittings are different sizes on true disc / drum systems.

If you have a big cammed engine, maybe you should look at a hydroboost system...

Have fun...

All good information! Also, there is a plug available that can be installed into the proportioning valve that keeps it from going off center while bleeding the brakes. I try to just gravity bleed the brakes after I bench bleed the master. Just put a hose on the bleeder screw and let it drip. Alternatively, I used to use a bleeder ball when I worked as a mechanic. Very seldom had trouble with the switch that way.

[AussieinNC]

This cutaway image shows the warning light switch and the shuttle valve that trips under circuit fluid imbalance.

I have witnessed these drum/drum master cylinders trip all over the place due to the imbalance between the disc circuit and the rear drum circuit...

This is why the disc trucks use a proportioning valve to balance the fluid pressures across the system.