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Spongy brake pedal

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by rampage, May 28, 2002.

  1. rampage

    rampage 1/2 ton status

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    I can't get my brakes to hold any pressure after bleeding them and wondering what the problem could be. I've been using one of those one man bleeder kits and bleed them until no more air bubbles of any sort are in the line. I do the brakes from rear passenger to rear driver then front passenger and front driver. I press the pedal down until it's about 2 inches from the floor hold it for a second or two then let up and continue that until there aren't any air bubbles in the line ... then I do it some more to make sure the line is clear. So far I've gone through 3+ quarts of brake fluid. The pedal is firm while bleeding but when I start the engine the pedal goes to the floor. New brake pads all around and new wheel cylinders in the rear and no leaks that I can see. If someone says it's the master cylinder I'm going to jump through my screen and bop you one /forums/images/icons/laugh.gif
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    When I tried to bleed mine, I couldn't get rid of a *spongy* pedal. Like you, I went through a lot of fluid, but it got to the point I just hooked a tube up to the bleeders, so I could re-use it lol.

    I lived with the spongy pedal for some time, then the brake light came on, then it started "walking" away sometimes at lights. I could still "pump" the brakes back up so they'd work, but I decided then it was time for a new MC, and it was.

    Bleeding ends up allowing the MC piston to move into the bore further than it did when it was already bled, so the seals apparently get torn up, and the MC no longer works effectively.

    Please don't hit me! /forums/images/icons/smile.gif
     
  3. rampage

    rampage 1/2 ton status

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    Hmmm, technically you didn't use those two words ... you're forgiven /forums/images/icons/laugh.gif
    I can't even get pressure by pumping them /forums/images/icons/mad.gif How can I tell for sure if it's the MC? I'd hate to replace that and find out it was something else.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    The way I look at it, right or wrong, is that the pressure your foot exerts has to go *somewhere*. Have you checked the rear shoes to make sure they are close to applying?

    Still sounds like the MC might be to blame, even if the brake shoes weren't adjusted tight, applying assist (booster) wouldn't change the feel, there would never be pressure.

    Not saying there aren't more things to troubleshoot, but when I was bleeding mine, and I knew the rear brakes were adjusted right, and that there were no leaks, I decided that I'd leave it as is because I couldn't be certain it was the MC. Of course later it got worse and I replaced the MC but I *still* wasn't SURE that was the problem, until after I did the swap and the problem was gone.

    I'm really not sure if there are tests for the MC ONLY, but if there is, it might be in the GM service manual, which I don't have access to right now : (

    If it makes you feel better, I think you can buy rebuild kits for the master cylinders...I always feel better if I at least have the excuse of wanting to find out how something works or goes together! Besides, anymore, even though a rebuilt unit may be extremely cheap, it might still not be put together right. A brake hone is about all that is "necessary" to rebuild one IIRC, although if the bore is fine, you could probably get away with a thorough cleaning...can't make it any worse than what you've got now.

    If theres a test specifically for the MC while it still on the vehicle, I'd like to hear it too.
     
  5. rampage

    rampage 1/2 ton status

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    When I put the new shoes on the rear I manually adjusted them until there was a slight drag. Then I backed up several times and applied the brakes to make sure they self adjusted, so I'm sure they're tight.
    I wouldn't rebuild the MC, I'll just get a new one if that's the prob. I think you just made me realize my worst fears /forums/images/icons/laugh.gif .../forums/images/icons/mad.gif
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    Why would that be your worst fear? If you've got the brake lines already bled, bench bleeding the "new" master cylinder and installing it is all thats required...

    Assuming (and thats a lot of ASSuME) that aftermarket MC's are built/re-built to original GM specs, and use the same seal material etc., I would have to say that replacing the MC *before* it fails in any case, really isn't that bad of an idea.

    In the long run, it doesn't cost that much money, and if you've got 10 years out of it already, (for example) and haven't been religiously flushing the brake system, a replacement really isn't that unwarranted. Kinda like changing the oil *before* the engine fails : )
     
  7. rampage

    rampage 1/2 ton status

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    Aren't they a PITA to replace? Never looked into it before but that's what I've always assumed.
    It's still the original MC, 13 yrs old ... so maybe a new one isn't such a bad idea.
     
  8. pr1aw

    pr1aw 1/2 ton status

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    Replaced a few MC in my years. Not to hard at all. My brakes are spongy also, and will probably replace the MC in a month or so. Every thing else is new.
     
  9. m j

    m j 1/2 ton status

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    fittings can be a bear sometimes.
    good idea to have a double flare kit and new fittings on hand.
    they will be rusted on.
    a shot of anti sieze on reassembly would make the job easier next time
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    As others have said, fairly straightforward..total of two bolts and two fittings off the top of my head.

    Definitely use the flare wrenches on the fittings. A few sharp rap's with a hammer (not a heavy blow that strips the flats) work well to free frozen fittings, and shots of some type of penetrating lubricant a day before attempted disassembly can either do nothing or help, but not hurt.
     

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