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Brake Booster/Master Cylinder Swap

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Zepplin, Sep 26, 2000.

  1. Zepplin

    Zepplin 1/2 ton status

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    I recently swapped in the brake booster and master cylinder from a 86 Chevy Truck into my 74 Blazer. Everthing bolted up perfectly on the firewall and under the dash where the pedal connects to the back of the booster. I haven't test drove it yet but, when I went to hook up the brake lines I had to put them in reverse order in relation to the old master cylinder. I already bleed the system but, compared to previous times when I replaced a caliper, etc. more fluid came out of the bleeder screw. When I bleed it this time only a little fluid came out. Does this newer style master cylinder have the front and rear fluid compartments reversed? If they are reversed compared to the old unit, then I should be o.k. but, worse comes to worse I will have to put in some new brake lines.

    I was just wondering if anyone else has successfully completed this type of swap. Thanks.

    Dazed and Confused in a 74 Blazer

    ZEPPELIN
     
  2. River

    River 1/2 ton status

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    It should not make any diverence how you conect the brake lines. One is right-front and left-rear, the other one is left-front and right-rear. This is done that when ever a brake line will leak you alway`s have a front and rear brake on opposite sides so can still brake and control the car.

    Hope this helped you little.

    Greetings from Amsterdam,

    River `90 K5, 350, TH700R4, NP241, "33, 3.08, 10 bolt F, 10 Bolt R(GOV-Lock)
     
  3. Zepplin

    Zepplin 1/2 ton status

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    Thanks River,

    I totally forget about the diagonal setup. I can't wait to try them out. Thanks again.

    Dazed and Confused in a 74 Blazer

    ZEPPELIN
     
  4. blazer72

    blazer72 1/2 ton status

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    Ahhh! Disc need more fluid moved then the drum so the piston is biger in the front chamber! as for cross braking split? Its a front rear split with the combo valve acting as a safety. the rear line run from one port on the combo valve do a frame rail in steal to a rubber line that drops down to the rear axel with a tee to each rear wheel.
    The fron comes off the combo valve with ether on lin that goes to a tee by the stearing box then to each wheel via a rubber line or it comes of the combo valve with 2 lines to each front wheel.
    The combo valve will shut of the front or the back system if a line fails at a bout the sametime the system runs out of fluid to give you one last good stop, the valve also turns on the light in the dash.

    The drum or rear line also has a stop valve in it that keeps about 10 psi in the line to keep the brake shoes closer to the drum. The front have a about similer valve that keep about 2psi in it. This valve can be in the master in the line seat or in the combo valve.

    I understand the jeep thing I drive a BLAZER!!

    Ken
     
  5. Scoobydoo

    Scoobydoo 1/2 ton status

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    I have never heard of a diagonal brake system??? This logic is crazy, if one system were to fail it would almost certainly cause you to pitch sideways, since the majority of your braking comes from the front brakes.
    The master cylinder is split front, and rear. And it works just like 72blazer said.
    That way if one system fails the other still works. And it will still brake in a straight line (just not nearly as well)

    The largest reservoir is for the disk. And I believe? you should also be using the proportioning/combo valve from the donor truck as well.
     
  6. talldogg

    talldogg 1/2 ton status

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    River, how's it going? I haven't seen you post in a while. Any update on that 4L80E swap? Take it easy.

    It's not what you buy, it's what you build!
     
  7. TX Mudder

    TX Mudder 1/2 ton status

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    Does anyone know for certain that the line sare diagonal?
    The reason I ask is that I swapped a 3/4ton MC into my 1/2 and the lines were like he said, reversed. So I put the front on the bakc and vice versa (On the hard lines on the MC).
    I thought one of the jobs of the combo valve was to delay (slightly) pressure to the front so that the rear has time to kick in. I don;t see how it would do this with a diagonal system.
    Come on, does anyoen know for sure? I've been wondering about this.
    -- Mike
     
  8. blazer72

    blazer72 1/2 ton status

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    there is a front system feed off the front bigger bore of the master and a rear feed of the small piston. Calling it a Combo valve is really not right nor is any type proprtioning valve though it get called both. What it reall is, is a safety valve and you should rev the lines if you do you will stop faster as the ft brakes will be work harder as the should. Haveing it the way you do still works just takes longer to stop and on snow or something slick the rear tend to lock up first. I run the master and combo out of a caddy that had 4 wheel disc as I have 4 wheel disc.
    You can run the stock disc drum stuff with all disc just take longer to stop. By the way I still run the stock 72 boster.

    I understand the jeep thing I drive a BLAZER!!

    Ken
     
  9. Brady

    Brady 1/2 ton status

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    I did this same thing on my '73 blazer.I used the booster and master cylinder off of an '84 3/4 ton 2wd.I think I used the hard line off of this as well because it was different.My brakes work great(disk drums).I am changing the rear to disks soon.I don't know how it will work with them reversed.isn't the front of the master cylinder bigger than the rear so it puts more fluid to the front brakes?I thought this was why we now have dual master cylinders along with safety if one fails you have the other to help stop you.Brady

    '73Beautiful Disaster
     
  10. River

    River 1/2 ton status

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    Everything is fine with me. Replaced my `89 K5 for a `90 K5.
    I have the transmission(4L80E) but it`s not so easy to fit it in a `90 K5. I`m in doubt if it`s worth the time and money to make the switch, it costs a lot to get the computer working.

    Greetings from Amsterdam,

    River `90 K5, 350, TH700R4, NP241, "33, 3.08, 10 bolt F, 10 Bolt R(GOV-Lock)
     
  11. Smitty

    Smitty 1/2 ton status

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    I had to replace the master cylinder on my '82. The cylinder I got had a plastic reservoir instead of being a one piece cast unit. I checked with 3 different stores since my truck had a cast unit but they were all the same. I'm not sure how good the brakes were with the cast unit since it was bad when I got the truck. The brakes work ok with the plastic unit but they aren't great. I had to reverse the lines and have thought this was part of reason for not having great brakes. I always thought the larger line went for the disc brakes and the smaller for the drum brakes on a disc/drum set-up. I'd never heard of one front and one rear working off the same piston in a master cylinder. Sounds strange to me, especially if the line sizes aren't the same but I've run across some pretty strange stuff on some of these trucks. Guess that's one of the reasons I'm addicted to this site, always learning something new. Now I'm going to have to crawl under and trace the lines so I can make sense of this new info.
     

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