Brake light stuck on?

Bman · Jul 17, 2014

Hi. I have a 1983 GMC sierra classic 1500. and the BRAKE light is stuck on. Ive checked the forum and read a bit about this.

Well in my case it was on when i got the truck. The brakes at that time were good, but alittle low on fluid, and the parking brake was held to the release lever with a piece of wire.

At first i assumed the parking brake just didnt work and that was the reason for the light. but i undid the wire and tried them. they work good. the brake lever does hang slightly.

The brakes them selves i put a new lid on the master cylinder (it was cracked) and topped it off.

Now im wondering where i should start looking. ive held the parking brake up but it didnt take the light off.

any ideas?

bucket · Jul 17, 2014

The switch on the park brake pedal may be broken.

Usually the light comes on when the brakes need bled, there's a leak, or the rear shoes need adjusted. It's less likely, but the pressure switch (that triggers the warning light) in the prop valve could have went bad.

350runner · Jul 17, 2014

Try the brake light switch on the brake pedal. It might just need to be adjusted.

bucket · Jul 17, 2014

350runner said:
Try the brake light switch on the brake pedal. It might just need to be adjusted.

Not the brake lights themselves, the BRAKE light on the dash.

350runner · Jul 17, 2014

That is a lot better lol

Bman · Jul 17, 2014

Alright thank you all. ill check all this tomorrow. From what i can tell the brakes aren't leaking. the level in the master cylinder hasnt changed since i topped it off.

HotRodPC · Jul 18, 2014

It's very uncommon for the proportioning valve to go bad, although many times people will replace it thinking it is bad and because the light went off after replacing it, then they think that confirms it was bad. Not true.

As bucket said, try bleeding the brakes. If you don't want to bleed them all, and you have good pedal, just bleed the fronts starting with the Right front, then the Left front. This will cause the prop valve to make a full swing one direction, then a full swing the other, then land where it's supposed to be. In short, resetting the proportioning valve and it should then be fine. If not, just make sure it's not the Park Brake switch or sagging park brake pedal. If you've bled the fronts good, and certain it's not the Park Brake, but the light is still on, then it's time for a new prop valve. I have a feeling the bleeding the fronts will fix it though.

chengny · Jul 18, 2014

Here is how to determine which switch (proportioning/combination or e-brake) is causing the brake warning light on the dash to illuminate:

Both of the switches are normally open. The lamp in the dash is always hot on the positive leg. When the contacts in either switch close, a path to ground is created on the negative side and the lamp lights up.

To determine which one is creating the path to ground - and lighting the dash warning - pull the harness connector from each switch (one at a time).

Do the prop/comb valve first - it's easier to get to.

You must be registered for see images attach

If the dash warning goes out, that switch is closed and you have identified the problem. If it stays on, plug the harness connector back in and go under the dash to the e-brake switch.

Pull it off

You must be registered for see images attach

- if the light goes out - that switch is the problem.

If it stays on, leave the harness connector off and go back to the prop/combo valve. Pull that harness connector off.

If the warning light goes out only with both disco'd - there is a problem with both switches.

If the light remains lit even with both switches unplugged - you have a short to ground in the negative leg of the lamp circuit.

87scotty · Jun 7, 2016

chengny said:
Here is how to determine which switch (proportioning/combination or e-brake) is causing the brake warning light on the dash to illuminate:

Both of the switches are normally open. The lamp in the dash is always hot on the positive leg. When the contacts in either switch close, a path to ground is created on the negative side and the lamp lights up.

To determine which one is creating the path to ground - and lighting the dash warning - pull the harness connector from each switch (one at a time).

Do the prop/comb valve first - it's easier to get to.

You must be registered for see images attach

If the dash warning goes out, that switch is closed and you have identified the problem. If it stays on, plug the harness connector back in and go under the dash to the e-brake switch.

Pull it off

You must be registered for see images attach

- if the light goes out - that switch is the problem.

If it stays on, leave the harness connector off and go back to the prop/combo valve. Pull that harness connector off.

If the warning light goes out only with both disco'd - there is a problem with both switches.

If the light remains lit even with both switches unplugged - you have a short to ground in the negative leg of the lamp circuit.

Extreamly helpful thank you sir

highdesertrange · Jun 7, 2016

very good trouble shooting guide. well explained with pictures to boot. thumbs up chengny. highdesertranger

RMWILSO323 · Feb 6, 2023

HAS ANYONE HAD ANY LUCK FINDING A Proportioning VALVE

Scott91370 · Feb 7, 2023

RMWILSO323 said:
HAS ANYONE HAD ANY LUCK FINDING A Proportioning VALVE

You'd have better luck starting your own thread instead of hi-jacking one that doesn't have anything to do with a proprotioning valve.

84Squarebodlvr · Aug 31, 2024

chengny said:
Here is how to determine which switch (proportioning/combination or e-brake) is causing the brake warning light on the dash to illuminate:

Both of the switches are normally open. The lamp in the dash is always hot on the positive leg. When the contacts in either switch close, a path to ground is created on the negative side and the lamp lights up.

To determine which one is creating the path to ground - and lighting the dash warning - pull the harness connector from each switch (one at a time).

Do the prop/comb valve first - it's easier to get to.

You must be registered for see images attach

If the dash warning goes out, that switch is closed and you have identified the problem. If it stays on, plug the harness connector back in and go under the dash to the e-brake switch.

Pull it off

You must be registered for see images attach

- if the light goes out - that switch is the problem.

If it stays on, leave the harness connector off and go back to the prop/combo valve. Pull that harness connector off.

If the warning light goes out only with both disco'd - there is a problem with both switches.

If the light remains lit even with both switches unplugged - you have a short to ground in the negative leg of the lamp circuit.

Thank you for this awesome and well detailed answer.

Unfortunately my problem after doing this is both switches have issues. (The light only turned off after both were unplugged)

Any reasons for why this would be?

I think I can try replacing the e brake switch.

And then also try bleeding brakes for the proportion valve switch to see if I can reset it. What you think? Appreciate you

Brake light stuck on?

Bman

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bucket

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350runner

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bucket

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350runner

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Bman

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HotRodPC

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chengny

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RMWILSO323

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