Brake Warning Light GMC C15

[Mlcdc1]

Recently noticed the brake warning light on the dash of my 1977 GMC C15 did not light up when engaging key in on position. After reading other threads, I was able to activate the light by removing the wire from the proportioning switch and grounding it, which if I understand it right, indicates that the circuit is good. I then reattached the wire to the proportioning switch but the dash brake warning light did not light up. Does the proportioning valve switch ever go bad and is there a way to check it? I’d appreciate any input on what might be the issue. Thanks in advance.

 

[WebMonkey]

i have an 85 k20 manual trans.

my brake light does NOT come on with key on.

it used to come on and stay on all the time until i rebuilt the rear brakes and 'cleared/reset' the proportioning valve.

now the light comes on when i set the emergency/parking brake OR if there is a fault noted at the p-valve.

i 'think' that's the way it's supposed to work.

my choke light comes on with key on until the engine starts but that's it.

good luck

 

[Goldie Driver]

I wanted to check - my '80 the brake light comes on momentarily as you are starting then goes out once the truck is running. Just a bulb test kind of deal.

 

[RoryH19]

i have an 85 k20 manual trans.

my brake light does NOT come on with key on.

it used to come on and stay on all the time until i rebuilt the rear brakes and 'cleared/reset' the proportioning valve.

now the light comes on when i set the emergency/parking brake OR if there is a fault noted at the p-valve.

i 'think' that's the way it's supposed to work.

my choke light comes on with key on until the engine starts but that's it.

good luck

Agree, the brake light comes on with the e-brake set or if there is an issue and the prop valve pin is not centered.
If I have the e-brake off and turn the key on it doesn't light up.
However with the e-brake off and I turn the ignition to start the truck it comes on.

Is that what you are asking about?

 

[Mlcdc1]

I appreciate the responses. No switch on the e-brake on a 1977 so think that doesn’t figure into it. My understanding is that when the ignition is in the on position or when starting the engine, the light should light up as a bulb check and then go off after engine is running. I get no light in either situation but as said earlier I can get the light to come on if I just unhook from the p-valve and ground it out so I,think that means the circuit is good. Seems as if something in the p-valve or the p-valve switch is the problem.