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Proportioning Valve (Again)

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by smack, Oct 18, 2002.

  1. smack

    smack 1/2 ton status

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    Okay, I changed my MC and have been bleeding for three days now but the pedal's still mushy. I know it's just a matter of time before I get all the air out and yes I bench bled the MC. But.....
    When I bleed, the fronts really flow the fluid but the back barely comes out.
    In light of the recent discussions of bypassing the prop valve (this would be difficult for me 'cos I've got a frame brace I'd have to remove just to get to it), does such low rear pressure sound like a bad valve to anyone?? I'm thinking it is and if I just got a new one my low-pressure at the rear problem might be solved.
    I have been unable to press in the button on the valve while bleeding and according to previous posts on this subject, that doesn't really make a difference anyway. Maybe it would in my case??
    Should I spend the money and replace the valve or just bypass the damn thing all together?
    HELP!!!
    smack
     
  2. TopOff

    TopOff 1/2 ton status

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    Do you have the secondary proportioning valve before the rear brakes? I have seen this mostly on the trucks. This is the valve that opens more as there is more load in the bed. It is mechanically connected to the axle and the bed frame. As the bed moves down this valve opens more and allows more pressure to the rear brakes.

    How about some other aftermarket valve. I have a JC Whitney manual valve on my rear brakes. This has helped me with my rears from locking up before my fronts.

    In any case, you should have more the same apparent pressure/fluid coming from the rear, as you do the front. At least I do on my '73 and my '71 truck.

    Maybe you still have air in the rear lines.

    Good luck.
     
  3. smack

    smack 1/2 ton status

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    No, pretty sure I don't have a secondary valve.
    An option would be to buy an adjustable valve for the rear but that just limits rear pressure, it doesn't increase it so basically my problem would get worse... I think.
     
  4. TopOff

    TopOff 1/2 ton status

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    Sounds like it would make it worse.

    I am in the same boat as you, My proportioning valve is in the front of the frame-rail. I am not sure how to "push the button" to bleed the brakes. I might also have to "bench bleed" my master cylinder.

    Maybe your rear drum cylinders need to be rebuilt or replaced???
     
  5. smack

    smack 1/2 ton status

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    Nah, I don't think so, I'm pretty sure the bleeder is like a T fitting in the sense that fluid splits either out of it or to the piston.
     
  6. txbluethunder

    txbluethunder 1/2 ton status

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    pull the rear drums off and adjust the auto adjust manually until you can barely slide the drums back on. then try bleeding you brakes. i had the same issue as you and it was the fact that my auto adjusters were rusted up and the back brakes were barely contacting the drums.

    -breakdown-
    when you hit the brakes the prop valve senses the diff in pressure between the front and rear brakes. it will not allow pressure to the front until the rear brakes have made contact and are up to a certain pressure. if the adjustors are too sloppy than it will take all of your brake pedal travel to get the back brakes to make contact and then the prop valve releases teh front pressure only giving you about 1 inch of brake pedal travel applied to the front pads.

    hope this helps
     
  7. imiceman44

    imiceman44 1 ton status

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    I had the same problem with my 73, it turned out that someone had resufaced the drums and didn't get oversize shoes, so the adjusting wasn't enough and the pistons would go too far out and suck air.
    I got oversize shoes and adjusted it and never had the problem.
    Point is, rear brake adjustment is always something that is forgotten in problems like this, check it before going any further.
    /forums/images/graemlins/thumb.gif
     
  8. smack

    smack 1/2 ton status

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    Yeah, but adjuster or not, shoes or not, shouldn't the pressure coming out of the bleeder be strong???
     
  9. imiceman44

    imiceman44 1 ton status

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    You also want to check the last rubber hose going down to the axle.
    I once had that one clogged.
     
  10. txbluethunder

    txbluethunder 1/2 ton status

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    mine wasn't.
     
  11. smack

    smack 1/2 ton status

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    Well, I can check that but moral of the story is nobody here thinks I have a bad prop valve?
     
  12. imiceman44

    imiceman44 1 ton status

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    Well if you take off the tube from prop valve towards the back and see that you still don't have enough pressure then yes.
    If you have pressure then you have a clogged tube or hose.
     
  13. smack

    smack 1/2 ton status

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    And you've never had a problem locking the fronts with the low pressure in the rear?
     
  14. txbluethunder

    txbluethunder 1/2 ton status

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    no cause the prop valve won't allow that to happen. just a side note...when your pumping the pedal then releaseing the drain valve on the wheel cylinder...is the brake light coming on? if it is than the prop valve is working fine...if its not than maybe you prop valve is bad???
     
  15. smack

    smack 1/2 ton status

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    Hmmm, that's another thing, absolutely no brake light whatsoever. Just figured the bulb was burned out or something.
    The switch is connected, but even when I step on the Park brake the light doesn't come on. Must be a bulb.
     
  16. txbluethunder

    txbluethunder 1/2 ton status

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    i'd say its the bulb then. if you get someone else to press the brake and get down next to the front crossmember (location of the prop valve) you can usually hear it click to one side. i would definately try hand adjusting the rear brakes and then spend the money to have the system power bled. that made a world of difference in my truck. from what i'm told its hard as hell to get all the air pockets out of the prop valve by manual bleeding.
     
  17. smack

    smack 1/2 ton status

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    Well I'm not one to bow down and admit defeat but I think maybe in this case...power bleeding-- I like the sound of that!
     
  18. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Front brakes (disc's) are about 900psi ) rear brakes {Drums) are like 300psi after the proportiong valve.
     
  19. smack

    smack 1/2 ton status

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    Well, okay, I'll just keep doing what I'm doing and will let you all know if it ends up working.
    Thanks,
    smack
     
  20. DieselDan

    DieselDan 1/2 ton status

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    Ahh, do you think maybe you got a bum MC? /forums/images/graemlins/blush.gif It sure would seem more likely than the combination valve suddenly going bad.
     

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