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Stock Proportioning Valve Ques

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by wakedog, Dec 6, 2001.

  1. wakedog

    wakedog 1/2 ton status

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    My rear brake arent working at all and the red brake light is on on the dash. It has happened before and it was low on fluid so i would add and they would be fine and the light went off. They probably need to be bleed(and they will this weekend) but where is the switch on the prop valve?? And i heard the switch will reset after they are bleed, true or not true?? Thanks

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  2. wakedog

    wakedog 1/2 ton status

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    come on

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  3. NE79

    NE79 1/2 ton status

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    Not sure, but mine's been doing that for 3 years now. Sometimes there's stopping, sometimes not. Kinda scary.

    uh-oh
     
  4. Pure Insanity

    Pure Insanity 1/2 ton status

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    I am not a brake expert but it is POSSIBLE that the master is worn out and bypassing. It will not have enuff pressure to turn off the lite, and can be intermittent.

    If ya can't stop.......Smile as you go under! [​IMG]
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  5. Triaged

    Triaged 1/2 ton status

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    There is a button on the front of the prop valve that needs to be pushed in to bleed the rear brakes. I use a dowel cut to fit against the rad. support. The light should go off as soon as the front and rear pressures are equalized. If you know you have air in the rear lines you might want to start bleeding at the prop valve and working your way back.

    '71 Blazer CST w/ a 400sbc, 4" lift, 36" Supper Swampers, and alot of rust
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  6. Goose

    Goose 1/2 ton status

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    Disc brakes activate almost immediately when applied. They ride very close to the rotor. Drum brakes activate slower because they ride farther from the drum. One of the things the proportioning valve does is to delay front brake engagement slighty to give the drums time to contact. Of course the valve also sets the brake bias. (typically 30% for each front tire and 20% for each rear) Lastly there is the sending unit which makes the brake light turn on when there is uneven front to rear bias. It sounds like you may have a rear slave cylinder leaking. The switch you need to press on the master cylinder is on one end under a rubber cap. This will allow maximum brake fluid to the front brakes when you bleed them. In theory, if you start with the rears, and bleed them good, you won't need to push in the switch when you do the fronts.
     
  7. DieselDan

    DieselDan 1/2 ton status

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    Goose is "on the money" here; thats why it's really called a Combination Valve. It's my understanding that the button only needs to be depressed when Pressure Bleeding, and is not neccessary (to depress) when manual bleeding. Yes, I would make another attempt (or 2) to bleed the brakes to extinguish the light. If it continues to return your master cylinder is need of replacement/rebuilding.

    Real trucks don't have spark plugs!
     
  8. Bigmeats

    Bigmeats 1/2 ton status

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    i had the same problem with my brakes and could not get them to bleed manually. I had to get them power bled to reset the prop valve

    Bigmeats

    1975 Blazer
     
  9. Leadfoot

    Leadfoot 1/2 ton status Premium Member

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    What method did you use for bleeding? Did you try bleeding the front or rears first? The reason I ask is I had done countless manual bleeds (with a helper in the rig and me yelling to them to step on and hold the brakes, me cracking the bleeders, closing the bleeder, yelling back helper to take foot off brakes, having them pump them up, and repeat until no air was detected), starting from furthest away from prop valve to closest, and never had a problem. I'm wondering if we do it differently, or if I have been lucky.

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  10. californiak5

    californiak5 1/2 ton status

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    I had the same problem on my truck. It turned out that the Master Cylinder was not holding presure. I changed it out and I haven't had a problem since.

    See Ya,
    Keith

    <font color=green>Born to Drive My K5</font color=green>

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