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Proportioning valve mod for rear disks?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Chris Demartini, Dec 27, 2001.

  1. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>I was looking through an old 4x4 mag and found an article on Jeep CJ rear disks swaps. They had <a target="_blank" href=http://members.aol.com/dawsonheath/chris.jpg>this picture </a> and told you how to get more fluid to the rear (since calipers require more fluid than wheel cylinders) by taking out the spring and the seal. Would this work on a GM proportioning valve?
     
  2. 2Dogs

    2Dogs 1/2 ton status Premium Member

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    Linky no worky.

    Dave

    Horn Broken... Watch For Finger.
    <a target="_blank" href=http://coloradok5.com/gallery/Daves-Pics>http://coloradok5.com/gallery/Daves-Pics</a>
     
  3. Scooter

    Scooter 1/2 ton status

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    Cant see the pic .. I keep getting this error ..

    We're sorry!
    The Webshots page you have requested has either moved or no longer exists

    It looks like you linked to the upload side instead of the actual image ..

    Scot in KC, MO
    Rough Country wont flex right? - <a target="_blank" href=http://coloradok5.com/gallery/albun29/aab>Pics</a>
    87 GMC 4" lift, 6 lug 14SF LockRite, 33s
     
  4. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>Wont load the image, I'm having it uploaded somewhere else now. These album things suck! [​IMG]
     
  5. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>Fixed
     
  6. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Most of the people on here (that do the swap) seem to do fine without touching it. If I did the swap, I think I would change it to an adjustable prop valve. I wouldn't do that mod. You still need to have more pressure in front than in the back since the front does more of the stopping AND you don't want to lock up the rear until after the front locks.

    Russ

    85 K30 CUCV, 350 TBI, TH400, 205, D60/C14, 4.56 Locked
    Some day: 4" lift, 44" tires, massive cutting, shorter wb and rear overhang.
     
  7. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>Yes thats true, but the pistons in the caliper are much larger than the wheel cylinders so you need more fluid to move them. I did this swap too, and I dont think I have enough rear brakes. I'm switching from eldorado calipers to stock 1/2 ton fronts, and if that doesnt help, I'll do this too. The adjustable prop valve would only decrease rear braking
     
  8. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    More fluid??? Man, I don't know about everyone else, but my 1/2 ton calipers will lock up that rear axle so fast - if I hit that pedal too hard in the rain, the rears lock, the rear starts to sway out... its just plain ugly. [​IMG]

    I need to install the proportioning valve in mine to calm down the braking power I acquired with the install of the rear disc. I would say screw the "modifications", and just get the new valve. You can get the <a target="_blank" href=http://store.summitracing.com/product.asp?d=4&amp;s=28&amp;p=1558&amp;SearchType=ECat>S U M M I T</a> model for $40. Why take the chance modifying your system, when you can get a finished product?

    <font color=red>GOT MUD???</font color=red>
    My license plate reads:<font color=blue> 8 YR SUV</font color=blue>
    454/TH400/NP205 - 14BFF/D60/w/ 4.10s - 36" TSLs
     
  9. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    I agree with BadDog too, you don't want the rears locking until after the fronts. My point? After bleeding my system with about 30 ounces of fluid, the truck stops on a dime on the (dry) pavement. When I romp the fat pedal in good dry conditions, all four lock up, and the truck just stops HARD.

    I had a scare once when I had my stock brakes. Thought I might not be able to stop before I was about to drive over this poor little unknowing-sedan... Would have been a masacre...

    Instead, I dumped it into the median, skidding completely sideways for about 50 ft. at 40mph. [​IMG] Needed a change of the pants after that little manuver! The truck took it great. I thought for sure that the thing was gonna tip on its side! 9" total lift, and the truck pulled off this lateral skid like it was a BMW. I just sat there in disbelief that I was still on all four tires...

    Enough of my rambling! Get the right valve, install it, bleed the HELL out of the entire system, and your truck will drop the hooks hard too!

    <font color=red>GOT MUD???</font color=red>
    My license plate reads:<font color=blue> 8 YR SUV</font color=blue>
    454/TH400/NP205 - 14BFF/D60/w/ 4.10s - 36" TSLs
     
  10. Chris Demartini

    Chris Demartini 1/2 ton status

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    <font color=blue>I wasnt going to do this until after I swiched to the 1/2 ton calipers. Bleeding isnt a problem, I'll just use a power bleeder at work
     
  11. DieselDan

    DieselDan 1/2 ton status

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    Has anyone else noticed the variety of results K5ers get after we make brake modifications? Hey, we're running different rigs, different tires, different lifts/suspensions, and different master cylinders. Even without the rear disc brake mod, I would plan on some brake system customization.

    My .02, If your rears lock up before your fronts: ditch the factory combination valve and use an adjustable proportioning. You don't need or want the metering part of the Combo valve with four wheel disc brakes. The proportioning part is not adjustable and, unless your lucky, is most likely not adjusted for YOUR non-stock vehicle. The brake failure sensor switch isn't anything more than what any of us (gearheads) can feel with our feet. If your fronts are locking up prematurely (and you think your rears aren't doing their share) use a bigger bore master (K30) AND an adjustable proportioning valve (for the rear).

    Real trucks don't have spark plugs!
     
  12. mnstr_fx

    mnstr_fx Registered Member

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    Taking out the spring and seal makes the proportioning valve into a diverter valve. You are not really sending more fluid to the rear calipers, you are just making it so that you do not have to over come an initial pressure before brake fluid starts to go to rear calipers. This enable more of the fluid to get to the calipers instead of being sent to front calipers.

    When I did my rear disc brakes, my brake line was TOAST! (Gee, a 77' in Illinois with some rust???) Anyway, I did what the guys in 4 Wheel &amp; Off-road did with there Dodge Ramcharger. Pitch all the lines and run two direct lines from the master to the calipers. Then just install a proportioning valve in the rears so you do not have them lock up on you too soon. So far, the system is easy to install and dirt simple.



    76-82 Chevy Pickup / Blazer - K5-K10-K30
    90' TBI
    TH 400
    203 / 205 Doubler
    Dana 60, Corp 14 - 4.88'd - Detroit Locked
     
  13. Twiz

    Twiz 1/2 ton status

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    Good point, we all run different combos. Everything isn't the same, but fairly close. I should probably keep my trap shut, since I haven't done the conversion yet, but that's never stoped me before, and it won't stop me this time!

    I know;
    -Disk brake systems need more pressure (force) to opperate correctly. Drum brakes are "self-energizeing", so they need -allot less pressure to operate. Proof: Back in the day, you could get a un-powerd (no booster) 4-wheel drum brakes.

    -Disk/Disk and Disk/Drum masters cyl are different. The disk systems should have a smaller bore, to add to the hydrolic leverage over the calipers. Like a hydro-jack or "pop-bottle" jack, the smaller the main cyl is the more addvanatge it has over the slave cyl. Hydrilic presses are also good examples of this, they commonly have two main cyl, one large one to take up the clearance, and a smaller one to applie the pressure.

    -Stock prop valves are designed for a Disk/Drum system, not a Disk/Disk system. They have a "quick-take-up" circut to over-power the rear Drum brake return springs. So, if the stock valve is used on a rear Disk conversion, then the rear brakes would applie under light brakeing, the fronts wouldn't even applie at-all, untill the pressure built up-enuff to by-pass the rear-bias. Not bad on paved (dry) conditions, but under slick conditions, the rear brakes would easily lock-up before the fronts would even think about moveing!

    -Some states require a Brake Warining Indicator to pass a safty. So, eliminateing this funtion, would also eliminate legal use on the street.

    I know Alot of people have completed this conversion, Solo-Wookie for example. I'd like to hear what they did, what they used (Master cyl, prop valve, etc.) and how they like it. What would they change.

    <font color=blue>Wow factors
    A SBC at 6,000 RPMs:
    2- The piston travels from O to 60 MPH to 0 in just .005 of a sec. and in just 7 inches.
     

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