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  1. #1

    Default Brake pedal goes to floor. Only when engine is running. Need some help.

    Hi, new to the forum. Just picked up a 91' K1500. Had to tow it home because the previous owner said the brakes didnt work right. Well he was right, they don't work. And now I need a little help with whats wrong.

    The brake pedal went to the floor when I picked it up. Noticed the rear lines were rusted through and replaced those. I flushed and bled the entire system. First using normal methods then I used a vacuum bleeder. I am confident in saying there is no air in the lines.

    With the engine off, the pedal feels very solid. I then start the truck and the pedal goes to the floor. I pump it several times with the engine running and it makes no difference in feeling. The pedal still goes to the floor. There is slight resistance when pushing down the pedal, but not much as it goes to the floor

    I then turn off the engine and the pedal is stiff again. Just like it was before I started the engine. I restart the engine and the pedal again goes to the floor. Engine off and it becomes stiff again.

    Any ideas? Vacuum booster? Master Cylinder?

    Thanks for the help!

  2. #2
    Jr. Engineer hundojoe's Avatar
    Join Date
    Feb 2010
    Elk River, Minnesota


    Vacuum booster is shot, but while you have it all apart replace the master cylinder also, its cheap insurance.
    1999 Silverado Regular Cab Short Box
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    5.3 with K&N CAI, Texas Speed 220/220 cam with 112 deg LSA
    Texas Speed computer Tune
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    4.11 gears front and back, with Eaton rear posi
    Flowmaster 40series true duals
    16X8 American Racing Nitros with 305/70/16 Goodyear MTR's
    285's in the pic

    If it ain't broke...........Give me a minute!

    Im not completely useless, I can be used as a bad example...

  3. #3
    murdog94's Avatar
    Join Date
    Feb 2008
    Cloquet, Minnesota, United States
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    Quote Originally Posted by hundojoe View Post
    Vacuum booster is shot, but while you have it all apart replace the master cylinder also, its cheap insurance.
    X2..... And you for sure want to do a master cyl since you will already have it off...


    1997 Chevy S-10 Blazer 4X4 with 330K miles and counting (Hunting rig).

    2009 Saturn Aura XE (wifes car)

    2011 F-150 Crew 4X4

    "Hold it to the floor till you see God.... Then Brake!!!!"

  4. #4


    thanks for the info. i'll replace them both while i'm at it.

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