brake warning light - proportioning valve

Discussion in 'Electrical Systems' started by Chuck71RS, Feb 26, 2018.

  1. Chuck71RS

    Chuck71RS Veteran Member Lifetime Gold Member

    1,094
    31
    Nov 24, 2012
    Houston, Texas
    With the engine running
    • brake warning light does not come on but front master cylinder reservoir is empty
    • Wiring terminal that goes on the valve connection shows 8.4 volts
    • Parking brake does light the warning light if the parking brake is not fully released.
    • Proportioning valve and parking brake wires are joined at the electrical plug on the firewall.
    Shouldn't the brake warning light come on because the front master cylinder reservoir is empty. Don't want to go through the effort of filling-bleeding the master cylinder if the proportioning valve needs to be replaced
     
  2. Twisted_Metal

    Twisted_Metal Moderator Staff Member Lifetime Gold Member

    25,752
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    Feb 26, 2004
    Bloomington, MN
    The light is controlled by grounding the circuit... Either through the parking brake switch or the switch at the combination valve. (It's NOT a low-fluid indicator.)

    The combination valve switch won't activate unless you put pressure on the pedal.
    If the piston force is unequal (between the two halves of the system) it slides the switch piston off-center and grounds the circuit to illuminate the indicator light.

    Normal position:

    [​IMG]





    Brake lights when the switch pin is grounded...



    [​IMG]
     
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  3. Chuck71RS

    Chuck71RS Veteran Member Lifetime Gold Member

    1,094
    31
    Nov 24, 2012
    Houston, Texas
    Thanks Terry. In reading the owners manual about the light, it said the brake light comes on if the system is....."partially inoperable", which I thought included low (or no) brake fluid.

    Should have added the brake pedal was pushed with the engine running and the light did not come on.

    Because the reservoir is empty pressure on both pistons is probably equal (zero) so the piston did not slide off-center. Do I have it right?

    Does uneven pressure at the rear brake drum cause the brake light to come on?
     
  4. Twisted_Metal

    Twisted_Metal Moderator Staff Member Lifetime Gold Member

    25,752
    443
    Feb 26, 2004
    Bloomington, MN
    Uneven pressure between the front and rear of the combination valve should have moved the switch piston and lit up the indicator.
    The valve could be bad or you could have air in the rear line and you are compressing air on both front and rear.
     

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