Proportioning valve questions

Discussion in 'Axles + Brakes' started by 90halfton, Mar 25, 2020.

  1. 90halfton

    90halfton I'm Awesome

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    Anybody ever seen this style of proportioning valve on these trucks? 90 c1500. Did the gmt800 mc swap and rwal delete few years back and just recently did the little shop rear disc kit. The rear wheels are barely locking up while on jack stands after thorough and proper bleeding. The drums did the same thing but I chalked that up to leaky wheel cylinders and a 30 year old rear rubber hose possibly being collapsed (I changed it out).
    Really has me questioning this prop valve. All of the ones I've seen have a port closer to the firewall for the rear proportioning side of the valve body. Little shops instructions mentioned disassembling the rear of the valve, removing the spring, and oring, and reassembling for better rear pressure but this is clearly a different kind of valve I cant even find in the Google. Eric the car guy on youtube even did a piece on modifying the valve in this fashion for a gmt400 disc and mc swap he did.
    There's also alot of conflicting information out there about the gmt800 disc/disc mcs not needing a prop valve due to internal design to accommodate the disc/disc setups they came with. And if a person were to essentially modify the rear proportioning section of the valve body wouldn't you essentially just be putting direct line pressure to the rear calipers? And therefore just leaving the valve body as a distribution block?
    I'm probably just going to install an adjustable proportioning valve or one intended for disc rears and be done with it. Anybody that has any insight or experience with this I'd appreciate the advice. I've emailed little shop about it but haven't heard back yet so I thought I'd run it by some of you that have wrenched on these for 30+ years 20200324_191154.jpg 20200324_191213.jpg 20200324_191217.jpg

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  2. delta_p

    delta_p OBS Chevrolet, When Silverado Was A Trim

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    They probably have you take out the spring and seal on the rear pressure reducer so it won't cut pressure to the rear in a panic stop since you went to disks and want the pressure now. Not the pressure cut so the drums won't lock up.

    https://www.gmt400.com/posts/1081027/
     
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  3. Gibson

    Gibson OBS Enthusiast

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    The idea behind the 400/800 series, regardless of disc/drum, or all disc, was to have a functioning anti-lock system, and let the abs system do its job,, no matter what the rear brake line pressure was.
    When people delete the abs their in vehicle they are just going back to the square body brakes,, even with rear discs.
    Adjustable valves are not a panacea,, will you re-adjust the valve every time you change the weight loading on the rear axle??,, it does make a difference in stopping power.
     
  4. delta_p

    delta_p OBS Chevrolet, When Silverado Was A Trim

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    I know on my '96, the abs pack is after the proportioning. And the abs sits idle in normal operation. When it activated, it takes over and isolates from the proprtioning and master cylinder with its own control of pressure. If I ever deleted the pack I would be adding back a separate combination valve like on the earlier models.
     
  5. 90halfton

    90halfton I'm Awesome

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    Well.....the abs has been long gone for years lol. And no. I will probably just adjust it as tight as they'll go without locking up on wet roads, or sand. And as far as everybody talking about adjusting them for different weights, what makes you think the factory system did that?
    Right now I'm just trying to assess the compatibility of an entire disc disc setup with this proportioning valve. I would like to keep it, but it's not looking like its gonna happen. And since I've never seen this type of valve in person or in any searches, and the rear drums never worked worth a shit even in good, well adjusted condition, I have to wonder if this isnt a leftover oddball gm threw on there. Owned it since new in 90.
     
  6. 90halfton

    90halfton I'm Awesome

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    My abs was downstream of the prop valve as well
     
  7. delta_p

    delta_p OBS Chevrolet, When Silverado Was A Trim

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    To be clear, what you and I are calling a proportioning valve is actually called a combination valve. It has 3 features when aggressively applying brake force:

    1. It has a front metering valve which holds off pressure to the front brakes only such that the rear brakes grab first. So the ass end doesn't come unglued.
    2. It the rear proportioning to cut the rear pressure so the rear drums don't lock.
    3. It has the dummy light (red brake light) mechanism.

    The do make combination valves specifically for disc/disc too.
     
  8. 90halfton

    90halfton I'm Awesome

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    Yes, by the look of if the front spring/plunger assembly is essentially a bladder that reduces uses the spring as a buffer to allow the rear brakes a slight head start. And the rear needle with the groove for the switch and the 2 orings serves as a pressure loss switch activator thingy. The oring closest to the spring and plunger keeps front and rear pressures separate and I'm assuming the rear oring is just there to keep it centered and not wobbling around? Idk. It's a different style of valve than I've seen before and different that the one in your link
     
  9. arrg

    arrg I'm Awesome

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    I have that style on my 88. 20200327_141457.jpg GM must have changed the design some time in the first few years of production.
     
  10. 90halfton

    90halfton I'm Awesome

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    Figured. Never saw pics but the valves the gen3 guys talk about sounds simila . The way I see it is it cant possibly be leading to poor rear brake performance. Gonna start back at the basics and rebleed and bleed some more
     

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