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Brake Warning Light GMC C15

Discussion in 'Brakes' started by Mlcdc1, Jul 4, 2020.

  1. Mlcdc1

    Mlcdc1 Junior Member

    Posts:
    9
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    Joined:
    Aug 19, 2016
    Location:
    IN
    First Name:
    Matt
    Truck Year:
    1977
    Truck Model:
    C10
    Engine Size:
    350
    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.
     
  2. WebMonkey

    WebMonkey Full Access Member

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    240
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    350
    Joined:
    Jan 3, 2020
    Location:
    Missouri Ozarks
    First Name:
    Chris
    Truck Year:
    1985
    Truck Model:
    K20
    Engine Size:
    350
    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
     
  3. Goldie Driver

    Goldie Driver Full Access Member

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    Joined:
    Mar 19, 2018
    Location:
    Houston, Texas
    First Name:
    Britt
    Truck Year:
    1980
    Truck Model:
    GMC K1500 Suburban
    Engine Size:
    350
    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.
     
    Blue Ox likes this.
  4. RoryH19

    RoryH19 Full Access Member

    Posts:
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    Joined:
    Aug 21, 2015
    Location:
    Texas
    First Name:
    Rory
    Truck Year:
    1987, 1982, 1983
    Truck Model:
    R10 Silverado SWB, K5 Silverado Blazer, K5 Silverado Blazer
    Engine Size:
    350 TBI, LS Swap 6.0, 350 carb
    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?
     
  5. Mlcdc1

    Mlcdc1 Junior Member

    Posts:
    9
    Likes Received:
    0
    Joined:
    Aug 19, 2016
    Location:
    IN
    First Name:
    Matt
    Truck Year:
    1977
    Truck Model:
    C10
    Engine Size:
    350
    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.
     

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