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Rear disc brakes on my 87 v20

Discussion in 'Brakes' started by dajn37, Feb 10, 2019.

  1. dajn37

    dajn37 Full Access Member

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    I started this conversion awhile back. I have searched and read a few threads about these conversions. My understanding is the proportioning valve needs to be changed due to the difference between drum and disc. Is that correct? I just thought I would start a thread about it and report progress. The conversion was simple. I have everything on and have bled the brakes but the pickup is still along way from road worthy so we will see how it goes. Once the weather gets halfway decent i am going to focus most of my efforts too getting my 3/4ton road worthy. I have had it over 2 years now and still haven't got it ready for the road. Just get to busy and sidetracked. I also put new shocks on the back and changed the rear differential oil while doing the swap.

    20170806_172557.jpg
     
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  2. dajn37

    dajn37 Full Access Member

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    Another pic. Apparently that's the only two I took.

    20170805_205313.jpg
     
  3. Frankenchevy

    Frankenchevy . Supporting Member

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    I ordered a disc / disc prop valve from summit. Can’t tell you the exact p/n, but it’s probably in my build thread somewhere.

    From what I’ve gathered, the drum prop valve keeps more residual pressure than the disc caliper wants and can cause your rear rotor to warp. Besides the other pressure and volume differences between the two.

    I can’t report as of yet on the new setup, except that the pedal pressure is firm—even without the hydro boost going.
     
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  4. 77 K20

    77 K20 Full Access Member

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    First of all in your picture your bleed screw is pointed down. You won't be able to get the air out of the system well like that.

    I researched and researched the whole rear disc brake thing. Seems there is no one "right" way to do it. Some used all stock components- and claimed it worked fine. Others had issues of rear lockup, no rear brakes... pretty much every possible outcome and I was more confused than before.

    I tried all stock- had rear brake lockup. Bought a disc/disc master cylinder (actually 2 different ones- didn't like either). Rear brake lockup. Then bought an inline proportional valve- and now I can adjust it down where it brakes pretty evenly. Then went back to just a regular stock master cylinder. I like that pedal feel the best.

    Try what you have first, use your hand or an IR thermometer to see if your rear brakes are dragging and then do some panic stops on gravel and see what it does. Then go from there if you don't like it.
     
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  5. Snoots

    Snoots Supporting Member Supporting Member

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    X2!
    And Frankenchevy is right on about the Proportioning Valve. You HAVE to change it too. No 'ifs or ands' or it'll bite you in the butt.
     
  6. bucket

    bucket Super Moderator Staff Member

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    Do you know the bore size of your current master cylinder? Running the adjustable valve in the rear circuit, did you also delete the original prop valve from the front X-member?
     
  7. dajn37

    dajn37 Full Access Member

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    That's correct. Now I remember that being my first picture and apparantly my only pic I took. I just had to rotate everything. I will take new ones when I get it in the garage when the weather gets better. I had it together wrong the first go. I forgot about that. Like I say I kind of put the v20 on the back burner the last year
     
    Last edited: Feb 10, 2019
  8. 77 K20

    77 K20 Full Access Member

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    I ended up buying a disc/disc prop valve to replace the stock prop valve. It didn't help much. Still had rear brake lockup even on pavement. The prop valve I bought was a Right Stuff Detailing PV72 Disc Brake Proportioning Valve. This didn't work so I had to add a Baer adjustable proportional valve after it for the rear brakes.

    [​IMG]

    The first MC I used was a ACDelco 18M1887. Ordered this because someone else said it "worked awesome" and was for a vehicle with front/rear discs. Very hard pedal feel because the cylinder was a 1.3385826772". The reservoir was so large it hit the vacuum line connection for the brake booster.

    [​IMG]

    And I believe with this MC it had the port sizes reversed... so I had to swap the brake lines around to hook them up. The lid never properly sealed- as soon as the truck would bounce around brake fluid would start seeping out from the huge lid. So I went to the "corvette disc/disc MC" that SO many said worked awesome. It was the ACDelco 18M91. Bore size: 1.125" This one no matter how well I bled the brakes had a very soft brake feel with a long brake pedal travel. Didn't like the feel of it at all. Used this for a few years and finally had enough of it. Ordered a ACDelco 18M92. Bore size: 1.25" If I remember right this is the right MC for a truck of my year but with the larger rear drum option. So I was able to put the brake lines back to the original spots on the MC. The brake pedal feel is great- firm, but not too firm.

    Like I said- I was SO annoyed at this whole thing due to how many posts I found online that completely opposite of what someone else said that worked. And since I am no expert I consulted and did all the work with my friend who has been a certifed GM mechanic since the late 80's. The work was done correctly.

    So best thing I can say is just try it and see what you think. Then modify from there if you feel the need.
     
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  9. bucket

    bucket Super Moderator Staff Member

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    Good info! I'll stick with the stock cylinder in my Burb then, it's got the 1.25 for the 13 inch drums. I'll just use the stock prop valve too, and put an adjustable valve on the rear circuit.

    What might throw me a curve ball is I'll end up with the big 1-ton calipers up front. My JB7 front calipers will just get moved to the rear. I suspect the pedal feel might wind up a bit softer than yours.
     
  10. dajn37

    dajn37 Full Access Member

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    I appreciate that. Thanks.
     

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