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Master Cyl and booster

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by grendel, Jun 1, 2004.

  1. grendel

    grendel 1/2 ton status

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    Before I go and hack up a perfectly good mounting bracket, what are all you 4w disk'ers running for a booster and mc in your 1st Gen?
     
  2. Triaged

    Triaged 1/2 ton status

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    Some guys have had very good luck using a 69-72 3/4-ton booster. I think it is a double diaphram to reduse peddle effort.

    There is no reason to change the MC out unless you want to go to a lighter aluminum one or your's is just old (in which case a stock MC will do fine).

    If you want to change anything swapping to a 4-wheel disc combanation valve couldn't hurt. I am using stock booster, MC, and combo valve and it works just fine (I can lock up all 4 on dry pavement with 36" tires without having to stand on the peddle).
     
  3. grendel

    grendel 1/2 ton status

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    My master is pissing all over the booster, currently.

    I'd like something with a little more rear volume. I bought a 78 3/4 ton and it looked great with equal size cyl's, but wouldn't fit my booster. So I bought the matching booster and it comes with it's own bracket, which would need a bit of modification...
     
  4. Ryan B.

    Ryan B. 3/4 ton status GMOTM Winner

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    http://coloradok5.com/forums/showflat.php/Cat/0/Number/872712/page/5/view/collapsed/sb/5/o/all/fpart/1

    This should answer your question about which power booster to run...

    I'm running my stock master cylinder, and this '72 3/4 ton truck vacuum power booster.. with 4 wheel disks it works great.. will lock up 38's without a problem.
    By far the greatest improvement i have done to my K5's brakes was the 3/4 ton power booster... before i used to have to stand on my brakes to get the truck to stop.. just swapping in the 3/4 ton booster with my stock half ton axles made a world of difference. /forums/images/graemlins/peace.gif
     
  5. Triaged

    Triaged 1/2 ton status

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    Interisting...

    I looked up a 78 k20 w/ 350SB on napaonline.com and came up with a bore size of 1.3125". That is real interisting because every other year k20 that I have looked up in the past had the exact same bore as 1/2-ton (1.125").

    If you have way to light of a peddle effort (brakes too easy to lock up) and it is too spongy that might be the way to go.

    That MC will push more fluid then the 1/2-ton MC but it will also do it at a lower pressure.

    I don't have any problem with mine so I'm not going to mess with it...besides even if the booster was enough to make up for the difference in MC size your brakes would SUCK if the motor stalled (something that happens all too often to me /forums/images/graemlins/dunno.gif).
     
  6. wraenking

    wraenking 1/2 ton status

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    im bidding on a wilwood proportioning valve. is that the valve you speak of? i am about to do the disc swap, so i wantt o make sure it goes smooth
     
  7. Triaged

    Triaged 1/2 ton status

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    I have one of those valves...I just haven't got around to installing it yet.

    To make it work properly you should "gut" your factory combo valve to get rid of some of the features you will no longer need. There are screw in caps front and rear on it. All you have to do is pop them off, remove the springs that are in there and reassemble it. Then just put the adjustable valve somewhere (some people even put it so you can access it in the cab) in the line going to the rear brakes.
     
  8. grendel

    grendel 1/2 ton status

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    The only problem with that 78 3/4 ton is the brackets.

    They won't bolt without fabbing three new holes.

    The push rod length from MC to booster is also much different.

    I'll take pics tonight of both.
     
  9. Yukon Jack

    Yukon Jack 1/2 ton status

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    Didn't want to hear that as I just pulled a brake booster off a 78 3/4 ton and I want to use it on my 70 K5. Oh well, can make it work, just will take more than just bolting it on.

    BTW - where is the proportioning valve usually? I need to pull it off the 78 3/4 ton parts truck.
     
  10. grendel

    grendel 1/2 ton status

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    It doesn't seem too bad. Drill three holes and cut the bottom of the bracket off. I don't know if the pushrod from the 78 MC is the same length. They seem to be different. May have to use the 78 MC as well.

    TAKE PICS! I won't have to!
     
  11. Triaged

    Triaged 1/2 ton status

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    [ QUOTE ]
    valve usually? I need to pull it off the 78 3/4 ton parts truck.

    [/ QUOTE ]

    On our trucks it is right next to the MC.

    On 73-up trucks it is on the frame rail. The only difference between the 2 is that the 73-up uses the combo valve as a "T" for the front brakes and ours has a bleeder valve there.
     
  12. grendel

    grendel 1/2 ton status

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    [ QUOTE ]
    Interisting...

    I looked up a 78 k20 w/ 350SB on napaonline.com and came up with a bore size of 1.3125". That is real interisting because every other year k20 that I have looked up in the past had the exact same bore as 1/2-ton (1.125").

    If you have way to light of a peddle effort (brakes too easy to lock up) and it is too spongy that might be the way to go.

    That MC will push more fluid then the 1/2-ton MC but it will also do it at a lower pressure.

    I don't have any problem with mine so I'm not going to mess with it...besides even if the booster was enough to make up for the difference in MC size your brakes would SUCK if the motor stalled (something that happens all too often to me /forums/images/graemlins/dunno.gif).

    [/ QUOTE ]

    Well, the 78 K20 MC and the 72 K20 MC are the same part number, from Kragens. It's Cardone.

    Boosters are one digit off from part numbers.

    I'll post pictures tomorrow.
     
  13. grendel

    grendel 1/2 ton status

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    [ QUOTE ]

    On our trucks it is right next to the MC.

    On 73-up trucks it is on the frame rail. The only difference between the 2 is that the 73-up uses the combo valve as a "T" for the front brakes and ours has a bleeder valve there.

    [/ QUOTE ]

    I've been trying to figure this out. I got a part number for a PV from TonyP to use. It has multiple ports on it...

    From [ QUOTE ]
    TonyP:
    part # 1257225 (proportion valve...)
    it's from a P30-60 truck that was a one ton ether lunch or utility truck and had 4 wheel disk brakes, they didn't have one in stock for me to look at but the guy said it will work...
    I'm going to try to get to another Chevy or GMC dealer to look at one...


    [/ QUOTE ]
    I have it, I'll take pics and post em tomorrow as well. It's strange. There are 5 ports on it. I assume two input, 1 for the rear and two for the front? I have to figure out which ones are which.
     

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