Possible Brake Booster Issue?

Discussion in '80-Series Tech' started by Death Star, Mar 13, 2018.

  1. Death Star

    Death Star

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    I’ve been dealing with the spongy brake issue and I’m at wit’s end. I rear ended (lightly) someone last week. I stood on the brake pedal with both feet and not much happened. I just bought a Motive Power Bleeder and flushed the brakes.

    One thing, at idle, when I hold the brake pedal down, it loses pressure and goes down. It’s like the system isn’t holding pressure. There are no leaks. There was no air in the system when I bled it but the fluid was nasty. There was no change in the braking performance after the bleed/flush. The brake pedal still feels like it’s not holding pressure.

    Could this be the booster?
     
  2. Malleus

    Malleus

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    To be clear, did you follow the inspection procedure in the FSM?
     
  3. Death Star

    Death Star

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    Which inspection procedure? I don’t have the FSM.
     
  4. Death Star

    Death Star

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    Ok. Let me rephrase that. Has anyone had a similar issue? How was it solved?
     
  5. flintknapper

    flintknapper

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    'Spongy' feeling brakes is almost always air in the system or softened brake lines (or both). Your brake booster is a vacuum operated diaphragm that assists in providing pedal 'pressure'. So IF it were failing you would feel a 'harder' pedal, much the same as when your vehicle is turned off (no power brakes).

    Two areas that can be difficult to remove air from are the LSPV and your Anti-Lock Brake mechanism.

    Its worth looking at your brake lines (flex lines) as well. If they are original to the vehicle....chances are good they are swelling when pressure is applied.
     
    Last edited: Mar 14, 2018
  6. Death Star

    Death Star

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    Thank you. I though it was weird that upon first push of the brake pedal, it would be firm but then push to the floor, like it was losing pressure. Perhaps it’s flexing the lines. There is a slight “hissing” you can hear when pushing the brakes. Is that normal?
     
  7. Warpigg

    Warpigg

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    My brakes would be hard sometimes and spongy on other occasions and sometimes all in the same instance. But most note worthy was in park it would go all the way to the floor slowly so I replaced my master cylinder and that help tremendously. But I still have spongy brakes every now and then but the pedal doesn't go to the floor anymore. I have some OEM soft lines waiting to go on ethier when I blow one or when the ground isn't a muddy snowy soup.


    Some odd things can happen when a brake booster fails it can give you a hard pedal as someone else said or it can "over assist" as in you barley touch the pedal and you lock up the brakes. Sometimes it can also cause your vehicle to run rough since the booster works off engine vacuum. So that could be the nature of your "hissing". But where exactly are you hearing the hissing? And does it do it with the vehicle off?
     
  8. Tedward

    Tedward 95 LC SILVER Star

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    search mud for the brake booster test

    involves depressing the brake pedal with motor on/off
    waiting a specified time and depressing again
    and what not
    it's a pass or fail test

    one other thing is to service a system
    not to pin all hopes on one part of it
    meaning soft brake lines and the master cylinder may be in addition to a failing booster

    I'd start with the free and clean job of testing booster
     
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  9. Death Star

    Death Star

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    Thanks everyone
     
  10. SUMMIT CRUISERS

    SUMMIT CRUISERS SILVER Star

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    Simple test for the booster.
    Engine at idle, depress the pedal, if rpms lower then booster is bad. Continued use will eventually lead to complete booster/brake fail. This is also bad for the exhaust valve(s) associated with the booster vacuum port. That's what I've found with every bad booster.
     
  11. MoJ

    MoJ Moderator

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    If you’re having brake issues and are not seasoned at understanding and diagnosing the system you should take the truck to a pro. Otherwise there’s too much at risk for your fellow driver.
     
  12. madisonr34

    madisonr34

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    Hard brakes = booster problem
    Soft brakes = air in system or master cylinder failing
     
  13. Bambusiero

    Bambusiero SILVER Star

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    I had mid-speed off throttle "brake-fade" that turned out to be a leaky vac booster hose grommet seal.
    Like - cruising along a city street and slowing down for a red light.
    Suddenly, had to stand on the pedal to slow down.
    RPM's and vacuum at mild cruise is low to begin with, then you let throttle off and RPM's drop to idle.
    A small leak in that situation -> no brake boost.
    The hose grommet seal looked fine, but was a bit hard, and maybe just a little loose.
     
  14. jonheld

    jonheld

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    No. That is not normal and indicates a vacuum leak from the booster itself or the valve/grommet.
     
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  15. ajax1

    ajax1

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    you shouldn't have a hissing sound. Was my master it would do what you are describing, make sure your lines are good and no air
     
  16. gnarlytoes

    gnarlytoes SILVER Star

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    As stated above:
    Hard pedal = booster or vacuum leak.
    Soft pedal = master cylinder and/or air in system and/or soft lines.
    At least it's not like my '92 when I first got it. Everything was bad, but the pedal felt normal because the symptoms canceled each other out.
     
  17. Gabriel 71

    Gabriel 71

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    my bet is the master I just replaced my soft pedal feeling master with a new Asin master cylinder have the best brakes the thing has ever had since my ownership wow it will actually stop now. You know you are getting older when stopping a rig is more important than going fast.
     
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  18. MauricioR

    MauricioR

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    upload_2018-3-16_10-55-57.png

    Same thing applies to the Brake Booster.
     
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  19. Death Star

    Death Star

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    Thank you all. I ordered all new flexible hoses and master cylinder. I’m going to replace the rear calipers since I’ll have it apart and don’t feel like bleeding it a second time. I’ll keep you all posted on the progress. I replaced the front calipers this past summer.
     
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