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Adjustable prop. valve

Discussion in 'The Garage' started by 72THING, Aug 20, 2002.

  1. 72THING

    72THING 1/2 ton status

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    I've converted my '72 pickup to rear discs and my rear brakes lock up before the fronts. I think I need to plumb in an adjustable prop. valve on the rear line. Do I put it in before or after my factory prop. valve? Thanks for any help.

    Will
     
  2. Dieselmh

    Dieselmh 1/2 ton status

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    I'd say if at all possible replace your old one with the new one. If that's not an option, then I'd put it behind the old one.
     
  3. 72THING

    72THING 1/2 ton status

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    Would it hurt to run just my front brakes thru my stock prop. valve? Or should I eliminate the stock prop. valve completely by running the front brakes straight and the rears thru the adjustable prop. valve? Thanks.

    Will
     
  4. BigMac

    BigMac 1/2 ton status

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    I believe your stock valve does two things. It delays braking to the front brakes untill the rears build presure. It also controles the "praportioning". I wonder if the presure delay is even necessary after the disk conversion.

    I would either add the new valve after the old valve, or consider a master cylinder and valve from a 4 wheel disk rig. I really want the newer master cylinder and valve but mine is new so im going to test it with what I have.
     
  5. m j

    m j 1/2 ton status

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    first search

    the factory valve is a combo valve, it does a bunch of things and a few safety features are built into it

    if you choose to disable it make certain that your brake pedal can bottom the master cylinder, if it doesnt and you loose a line or wheel cylinder you will have no brakes at all in either front or rear. that is covered in the stock combo valve by the shuttle, it closes off the dead system to allow the master to generate pressure.

    all you out there that suffered the red brake light on the dash and unplugged the wire, you have no rear brakes. please hook the wire back up and fix the brakes.
    I hate seeing the red glow from the brake light on your faces when I am on the freeway with you
     
  6. BigMac

    BigMac 1/2 ton status

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    so... it does more than two things and Im not smokin crack thinkin I should leave it there....
    How bout updating to the 4 wheel disk valve?
     
  7. m j

    m j 1/2 ton status

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    you would have to find out exactly what differs between the 2 applications to decide if it is advantagious or necesary
    I think we determined the residual pressure valve was in the master in another thread so that wont change with the combo valve
    the hold off feature would have to be looked at but it only really applies to light braking IIRC.
    I really hate prop valves. setting them up for dry conditions will reduce your wet braking, and setting them up for wet will have the rears locking in the dry.
    it is a huge compromise, then you have to think about the loaded/unladen....hard to be worse then the factory I guess
     
  8. jimmy88

    jimmy88 1/2 ton status

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    Go to the 73-91 K5 forum 8/17/02 post by Thumper "Brake Prop Valve...Revisited and you'll see the description of the combo valve from the service manual. (sorry I don't know how to link forum threads) Looks like the rear disc performance will depend on a lot of factors, some guys have no problems, some do. I'd find a good place to buy the adjustable prop valve and pick their brains on the best way to install it. Brakes are kinda important to be guessing with, just my 2 cents.
     
  9. jimmy88

    jimmy88 1/2 ton status

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    mj, I've had a couple of the masters apart and have seen no residual pressure valve. I believe you are talking about the check like valve that typically screws into the master where the brake lines attach??? If so it looks like the "hold-off" section of the combo valve does that job now.
     
  10. 72THING

    72THING 1/2 ton status

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    Thanks for the help, guys. I think I'm going plumb the adj. prop. valve in after my factory "combo valve" when I finally get ready to do it. I found one in Summit Racing for about $40. I'll let ya'll know how it turns out when I finally do it.

    Will
     
  11. Emmettology 101

    Emmettology 101 3/4 ton status

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    I broke a front line on the trail once. I noticed it since the pedal pressure went down some. The broken line would keep pumping fluid out until I clamped it off and then had good pressure to the other 3 brakes.
    So what you are saying is that a stock prop valve is supposed to sense the broken line and shut that line off? I have broken lines twice and have seen other do it and none of them close off, they all leak fluid. And since they leak fluid, it wouldn't be long until you lost your brakes altogether since it would empty the MC...
     
  12. m j

    m j 1/2 ton status

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  13. jimmy88

    jimmy88 1/2 ton status

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    From the cut away drawing it looks like the combo valve just seperates the two halfs of the master cylinder. I guess in your case it would let the resevior that feeds the front brakes pump out while keeping the rear brakes working.

    Came across some good infocombo/prop valve link
     

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