REAR DISK CONVERSION....

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by Mmo, Jul 11, 2019.

  1. Mmo

    Mmo she aint pretty, she just looks that way Premium Member

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    hey fellas
    ive read a few of the rear disk brake conversion threads, re to swap or not to swap the master cylinder. ive a new one that came with my truck. putting el dorado calipers out back for the mechanical E brake option, wilwoods already clamping on the front disks.

    looking at my attached MC, any words of wisdom? we have to run the truck up on jack stands anyway to check for drive shaft wobbling, so will experiment with putting the brake on and see if they stop equal and quickly. then a test drive.... IMG_4489.jpg IMG_4488.jpg
     
  2. 6872xtc

    6872xtc make mine modified Premium Member GMOTM Winner

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    I'm not familiar with the company name cast into that master cylinder. So I can't help you. It seems to depend on what the pedal ratio is and the bore diameter of the master cylinder as to what works. Without knowing specific information, you may be in for some trial and error. When I converted my '70 K10 to rear disc, I ended up going to a Corvette master cylinder with hydro-boost. Part of my issue was trying to keep it tucked back out of the way of the A/C compressor, since I have it on the driver side. It stops fine now, but the pedal pressure feels somewhat high to me.
     
  3. Mmo

    Mmo she aint pretty, she just looks that way Premium Member

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    thanks man. i went ahead and bought the Summit master cylinder proportioning valve. seems a good piece of kit and only $90. haven't received yet, and will likely try without just to get a baseline. my limited understanding is that the smaller wheel cylinder for drums needs more pressure than the larger calliper piston. i believe that drum master cylinders also supply a little constant pressure to the rear as well; i reckon that's a problem for my impeding rear disks... gasp?
     
  4. bp71k5

    bp71k5 3/4 ton status Premium Member

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    Looks like you already have the updated proportioning valve? I don’t thing the master cylinder has the residual pressure valves in it, but the proportioning valve did. Might want to check me on that. The disc master cylinders are needed for discs cause of the increased volume of fluid they need for the self adjusting calipers.
     
  5. Mmo

    Mmo she aint pretty, she just looks that way Premium Member

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    this MC and proportioning valve kit was always on the truck, but new when i bought. but you're saying it a better way than I; it's the proportioning valve that keeps residual pressure for rear drum brakes. so not desirable on my impending rear disks.
    re your comment about disk MCs, this is due the brake pads wearing and the fluid level going down in the reservoir, the rear one being smaller on rear drums. um, ya?
    curiouser and curiouser
     
  6. Blazooki

    Blazooki Five-O Premium Member GMOTM Winner

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    Disc brakes require more fluid movement than drums to work. The factory proportioning valve can keep that from happening when you switch from drums to rear discs and is probably the biggest cause of issues when switching to rear dics. Most people remove the factory valve all together. They put in an after market adjustable valve inline and use that to adjust front/rear bias. like this one:

    https://www.jegs.com/i/JEGS/555/630...QobChMI37_DoqSw4wIVEdRkCh18sANwEAQYASABEgKmjf

    here's a good article on options
    http://www.fourwheeler.com/how-to/transmission-drivetrain/129-1103-14-bolt-disc-brake-conversion/
     
  7. bp71k5

    bp71k5 3/4 ton status Premium Member

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    Yes.
     
  8. bp71k5

    bp71k5 3/4 ton status Premium Member

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    If I was going to do it again, I’d also do it that way. The rear disc prop valve I’m not sure works all that well, or at least with bigger than normal tires.
     
  9. Mmo

    Mmo she aint pretty, she just looks that way Premium Member

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    Thanks fellas. Reckon the entire stock proportioning valve system comes out and i'll install the new one.
    i'll leave my new stock master cylinder in there and she how she stops.
    be starting next weekend....
     
  10. GMCPaul

    GMCPaul Registered Member Vendor

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    The master cylinder on your Blazer doesn't need replaced when adding rear disc's. The only thing that needs replaced is the proportioning valve. Currently you have a disc / drum and you need disc / disc. The prop valve is what has the residual pressure valve in it that kept a small amount of pressure on rear drum brakes. The master cylinder currently on yours is from Performance Online and is there 1" bore corvette style.
    Paul Jr @ GMCPauls
     
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