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Proportioning Valve Information

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Goose, Mar 24, 2001.

  1. Goose

    Goose 1/2 ton status

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    I was on the AC Delco sight looking for brake parts and came across some cool information about the brake proportioning valve...actually they call it a combination valve.

    The valve consists of :
    1) Proportioning valve that controls the front to rear brake bias.
    2) Pressure differential switch that senses pressure drop between the front and rear which activates the brake waring light.
    3) Metering valve that delays hydraulic fluid going to the front brakes until 30psi.

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    "The metering valve is used on rear-wheel drive vehicles with front disc/rear drum brakes. Disc brakes react quicker to intial brake application than drums due to tighter pad to rotor clearances. The meteing valve delays front brake application for a fraction of a second so the rear brakes engage at the same time as the fronts.

    In some instances, the meteing valve can prevent fluid flow to the front wheels during pressure bleeding. Therefore the valve may have to be disabled when pressure bleeding. Special tools are available to disable the valve by depressing the valve stem. From 4 to 30 psi, the metering valve remains sealed, closing off brake fluid from the front brakes. The master cylinder delivers brake pressure to the rear brakes only. Hydraulic pressure acts against the drum brake return springs to bring the brake shoes in contact with the drums. Avove 30 psi, the hydraulic pressure begins to overcome valve spring pressure, opening the metering valve seal." (paraphrased from AC Delco)
    So when the rear pads contact the drums, the hydraulic pressure raises quickly which overcomes the metering valve and lets fluid flow to the fronts. Since there is no air in the system and the fluid doesn't compress, it's pretty much instant.

    I thought this was good information because it actually explains why some manuals say to depress the metering valve stem and others don't mention it. It seems to me that if you bleed your rear brakes first and have them adjusted properly, you would not need to depress the metering valve stem. But if you just bleed the rears, and have not adjusted them yet, you probably would need to depress the metering valve stem.



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  2. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Thanks much. I knew that it created a pressure differential but I had no idea about the delay.

    Bad Dog

    85 K30 CUCV, 350 TBI, TH400, NP205, D60/C14, 4.56
    Coming soon: 4" lift, 40" tires, massive cutting, shorter wb and rear overhang.
     
  3. DieselDan

    DieselDan 1/2 ton status

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    It's my (limited) understanding that key word here is "pressure" bleeding. Pressure bleeding with bleeder ball style of pressurized brake fluid typically does not raise the pressure (sensed at the metering valve) enough to open the valve, especially when air bound. I personally don't like pressure bleeders because of the large amount of brake fluid that gets exposed to the air (moisture) and sits there long before it ever gets to the master cylinder.
     
  4. Blazer79

    Blazer79 1/2 ton status

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    Thanks a lot, Goose. That's some really valuable info you got for us.

    Do you know if they had a brake warning light back in 79?

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  5. Eric78K5

    Eric78K5 1/2 ton status

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    Thanks for the info...I'm going to change my rear drums to disc's sometime soon. I'm not sure which valve I'm going to need, but this info helps me understand what's going on.

    Eric78K5
    K5-Because Size Matters!
     
  6. Chewy

    Chewy 1/2 ton status

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    That's some great info. I am doing a brake job on my truck next week. Thanks for the help.

    '87 Beast
    Colorado, USA
     

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