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Removing the proportioning valve

Discussion in 'The Garage' started by TX Mudder, Dec 21, 2002.

  1. TX Mudder

    TX Mudder 1/2 ton status

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    My understanding of the valve is that it:
    1. Delays front brakes until 30psi in line (because discs grab quicker than drums)
    2. Senses brake failure and triggers dash light
    3. Controls front vs rear percentage of braking

    So, for a rear disc conversion there is really no need for this thing.

    Now, bending and flaring brake line is easy enough, but where can I find a splitter so I can send fluid to left front and right front?

    I plan to use a Wilwood valve for the rear.

    -- Mike
     
  2. Triaged

    Triaged 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />

    1. Delays front brakes until 30psi in line (because discs grab quicker than drums)
    2. Senses brake failure and triggers dash light
    3. Controls front vs rear percentage of braking



    [/ QUOTE ]

    #2 Is the reason to leave it in and just gut it. You also won't have to find a T for the front line.

    If you do go against my advice and ditch it...you can get a T at just about any parts store.
     
  3. TX Mudder

    TX Mudder 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />
    #2 Is the reason to leave it in and just gut it

    [/ QUOTE ]

    With some recent 'mods' to my dash, that light has been removed so it doesn't do me any good.
    My prop valve is a 1976 original and is the only piece of my braking system (other then some hard line) that hasn't been replaced.
    I don't need it.
    Thanks for the info about the T. I didn't think they would have that, but glad they do.
    -- Mike
     
  4. m j

    m j 1/2 ton status

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    does more then trigger a lite, also closes the broken system off to allow pressure in the master to keep it functioning.
    if you have a seperate master for each system or are 100% postive that you can bottom both pistons with the pedal then go ahead and remove it.
    If you are not 100% then do not remove it.
    in the event of a broken line you may end up with zero braking with out the shuttle valve in the system.
     
  5. chevyfumes

    chevyfumes Court jester

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    Watch for the muzzleflash!
    as always... /forums/images/graemlins/thumb.gifgood input MJ....
     
  6. m j

    m j 1/2 ton status

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    LOL
    you should be cautious making a post like that!
    you may gain a lot of enemies.
     
  7. chevyfumes

    chevyfumes Court jester

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    Watch for the muzzleflash!
    Well if not good input than interesting input none the less.... /forums/images/graemlins/thumb.gif
     
  8. TX Mudder

    TX Mudder 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />
    does more then trigger a lite, also closes the broken system off to allow pressure in the master to keep it functioning.

    [/ QUOTE ]

    I thought the idea w/ a dual-resevoir MC was that even if one side completely crapped out, the other side would sitll function normally.
    Am I wrong?
    -- Mike
     
  9. m j

    m j 1/2 ton status

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    this has been discussed here, you may have missed the thread.

    many dual master cylinders require hydraulic resistance between the primary and secondary pistons to build pressure.
    there is no physical connection between the pedal and the second piston other then hydrualic.

    now if you have tested, and can be 100% certain you will have brakes in the event of either system line failure this is a moot point.

    to test it you would pretty much have to remove the combo valve and then disconnect one line and leave it open to atmosphere and measure how much hydraulic force you can apply to the remaining system.
    then refill reservior and try the other system.
     
  10. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I had a rear line blow a couple years ago and lost my brakes entirely...so my faith in that combo valve isn't that strong. I was fortunate I was very near home and it was 2am. I shut the motor down and used a bit of compression braking to slow me down and eventually stop me. Another instance where I was very glad to have a SM465.

    Rene
     
  11. thebigdaddyof2

    thebigdaddyof2 1/2 ton status

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    I wasn't aware that this was one of the functions of the proportioning valve. A few years back, I ripped a stainless (extended length) front brake line completely off....I lost ALL braking- the pedal went to the floor and I kept moving. Thankfully, I was in the woods and was only rolling AND I had an E-brake at the time. If they do have this function, I have no faith in them.
     
  12. m j

    m j 1/2 ton status

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    so that kind of reinforces my point on the master cylinder.
    I have blown a rear line in a GM and had a long pedal but some brake function.
    brake hydraulic systems are usually very neglected
     
  13. DieselDan

    DieselDan 1/2 ton status

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    Ah, I think you need to re-read the (long) OFFICIAL DISC BRAKE POST. Alot of things were discussed. My understanding is the combination valve does not "cap off" one side if the other side leaks, it only tells you there is a pressure difference. And, if I'm correct, the master cylinder does have a solid mechanical link that will connect the two (front &amp; rear) pistons in the event of a fluid leak. But don't take my word for it, /forums/images/graemlins/rolleyes.gif
     
  14. m j

    m j 1/2 ton status

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    well i hope there is SOME correct info on the second page cause the first is fumbling in the dark...
    the master puts equal pressure on each piston in it. as they are both being activated by the drivers foot how could it be any other way /forums/images/graemlins/rolleyes.gif
    that hasnt been mentioned in an entire page of posts
     
  15. m j

    m j 1/2 ton status

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    didnt see a single word of anyone even looking to see if the shuttle valve traveled far enough to block a leaking port.

    perhaps you could cut n paste the section or at least provide the name of the poster that stated it.
     
  16. m j

    m j 1/2 ton status

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  17. ntsqd

    ntsqd 1/2 ton status

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    OK, I'll take a stab at this.

    Combo-valve: I've heard that same line that in the event of a pressure loss on one half of the system that the shuttle piston will move sideways, turn on the "Brake Failure"" light, and block off the failed side so that pedal travel isn't excessive. I've never seen one that would block things off, only turn on the light. Something tells me you won't need the light.....

    Master Cylinder: The front and rear pistons in a dual m/c are semi mechanically connected. There is a spring btwn them and a bolt that limits how far apart they can get, but there is no direct mechanical connection that drives both pistons simultaineously (sp?). This is deliberate, it's how the system compensates for the different take-up volumes of the front and rear systems. In normal operation the rear piston builds enough pressure to create a 'hydraulic lock' that moves the front piston directly. If the system the rear piston is trying to pressurize has a leak and won't build pressure, the rear piston will travel a short distance w/o affecting the front piston at all. At some point the rear piston will physically contact the front piston and force it to move forwards.

    For a long wheelbase I would give serious thot to the LBS: http://dpiracingproducts.com/productdet.php?producttype=Brake_Systems#DPILBS It doesn't seem to work as well on wheelbases under ~95" This is similar to the delay valve that GM used to use in early front disc systems, except that you put it in the rear brake system, not the front.
     
  18. m j

    m j 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />
    At some point the rear piston will physically contact the front piston and force it to move forwards.


    [/ QUOTE ]

    assumes that you have enough pedal travel to bottom the master.
    is this the case?
    so far noone has proved this that i have heard.
    in fact Rene just posted he was unable to get any brake pressure after breaking a rear line, which is 3rd party anecdotal evidence at best, but does lean me toward perhaps it requires a test to ENSURE that you will be able to have brakes if you start pulling "unneccesary" parts that GM decided to put in the vehicle
     
  19. DieselDan

    DieselDan 1/2 ton status

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    Yes, with a doubt, there is alot of "theory" on the OFFICIAL BRAKE POST" /forums/images/graemlins/grin.gif That what makes this site great, when you start to question what you've always taken for fact. (Right now I don't believe anything, including myself /forums/images/graemlins/blush.gif

    The link you posted to Dorian's site was interesting. But that shot is the metering valve portion, what's that prove /forums/images/graemlins/confused.gif Are you telling me the metering valve "caps 0ff" fluid in the event of a line failure? That won't do much for rears will it /forums/images/graemlins/smirk.gif

    Maybe this link is a little clearer COMBINATION VALVE Interestingly nowhere on the Delco site Dorian referenced does it mention "capping off" brake fluid. Funny, there is no mention of that function in any of my GM service manuals either .

    Hey, take me with a BIG grain of salt MJ. (I don't want to sound like the BIG HEAD). I've just never seen this function of the Combo Valve in any official pub. I've heard people talk about it but /forums/images/graemlins/ignore.gif

    I'm gonna have to start looking around for some of the old MC and Combo Valve cores we used to have laying around. Maybe I can get someone to slice a valve in half for some pictures. All in the name of Science /forums/images/graemlins/rotfl.gif
     
  20. m j

    m j 1/2 ton status

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    </font><blockquote><font class="small">In reply to:</font><hr />
    I'm gonna have to start looking around for some of the old MC and Combo Valve cores we used to have laying around. Maybe I can get someone to slice a valve in half for some pictures. All in the name of Science

    [/ QUOTE ]

    that would be cool.

    what would be better would be if someone would check if they can bottom the master and have it build pressure enough to stop the truck on each system individually.


    my bad on the link, niether illustrates my point
     

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