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Thread: Modifying Stock Proportioning Valve

  1. #1
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    Question Modifying Stock Proportioning Valve

    I'm converting my brakes in my '87 over to 13" cobras up front and 11.65" discs in the rear. I have purchased the '95 cobra booster and 15/16" master cylinder with 2 outlet ports instead of three.

    I have been reading all the posts about gutting the proportioning valve to make the 3 to 2 conversion. I have purchased the FRPP adjustable proportioning valve for the rear brake line. My questions is this: What is the purpose of the stock proportioning block once it's gutted? More specifically, could i run a single line from the rear port of the new master cylinder to a brass "T" fitting and split the line to each front wheel? Then the line from the front port of the master cylinder will go through the FRPP adjustable proportioning valve on it's way back to a "T" on the rear axle before being split. I will still be able to adjust the front to back bias and each front brake will receive equal pressure from the rear master cylinder port.

    Is there a reason I can't eliminate the stock valve altogether and go with the above described setup?

  2. #2
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    Eliminate it, and just install your adjustable one. That's what I did.
    1987 Tbird 5.0 swap, go fast mods coming soon....

  3. #3
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    gumby - beauty may fade, but stupid is forever!

  4. #4
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    Quote Originally Posted by DIAF View Post
    I have been reading all the posts about gutting the proportioning valve to make the 3 to 2 conversion. I have purchased the FRPP adjustable proportioning valve for the rear brake line. My questions is this: What is the purpose of the stock proportioning block once it's gutted?

    Is there a reason I can't eliminate the stock valve altogether and go with the above described setup?
    After gutting the proportioning valve in the combination valve, the only piece left is the shuttle valve. In '87+ MCs it is believed the only function is to eliminate rear proportioning (give full pressure to the rear) in case one loses fluid pressure to the front brakes. Since you have already eliminated the proportioning part of the valve, the shuttle valve is essentially useless. Thus you can remove it without affecting anything.

    Pre '87 shuttle valves are believed to isolate (partially at least) the failed circuit to provide pressure so that the surviving circuit gets pressure before the piston bottoms out. In this case, leaving in the shuttle valve adds safety. In actual practice, the valve doesn't always work. Take a look here to see how the MC operates.

    From a liability standpoint, removing the proportioning valve could land you in hot water if you crash into someone due to brake failure since you have purposely removed an OEM safety feature.

    Excellent shuttle valve discussion here.

  5. #5
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    Quote Originally Posted by gumby View Post
    That is exactly the setup i was talking about going with. Thanks.

  6. #6
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    My set-up on the '80 is similar to gumby's (no stock valve at all just a T) and I used the larger Wilwood adj prop valve.
    Long live the 4-eyes! - '83 Tbird Turbo

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