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No brake boost at idle

Discussion in '6.9L IH & 7.3L IDI Diesels' started by lkrasner, Apr 18, 2019.

  1. lkrasner

    lkrasner Registered User

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    Well pretty sure my vacuum pump is dying, just looking for some more sure diagnosis. Brake light has flickered since I got the truck, but brakes always felt great. Today, they started getting hard at idle. They are fine coming to a stop from speed, but if I then pump it a couple times, there's no boost. It slowly comes back, and if I get on the throttle it comes back faster( in park, I can pump the pedal till its hard, then Rev it a bit and fee it sink back down like it's building boost).

    Now, I just threw a serpentine belt after getting oil all over it (I need a funnel...), and replaced it with a house brand belt from O'Reilly. It was a bitch go get on, but didn't really seem to short or anything. I cleaned all the pulleys before putting the belt on, and it doesn't look like it was slipping. The pump felt good rotating by hand when the belt was off. I did also notice a decent amount of oil coming out the top of the pump. Is this a sign of failure? Is there any way to rebuild these things?
     
  2. FordGuy100

    FordGuy100 Registered User

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    Sounds like a classic vacuum pump failure to me as well. Take the belt off, take off the vacuum hose and rotate the pump by hand with your finger on the outlet. Should build good vacuum and hold it after a revolution or two. If its bad you can spin it over and it will build some vacuum but nothing to write home about.
     
  3. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    Yep. Sometimes the serpentine belts can be a SOB to get on. I'd say that it sounds like a vacuum pump to me too. I'll second the "finger test". You can also buy a vacuum gauge to keep an eye on the vacuum. I love the fact that I installed one in my truck. I think it's very nice to have.
     
  4. lkrasner

    lkrasner Registered User

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    Great, so I get to muscle that belt off again haha. I'll give it a shot in the morning. Driving it 3 hours tomorrow, would rather not loose all brake boost, it's a bus and it's heavy...

    Do you think a junkyard pump is a good idea, or should I try to find a new one?

    And what's all this about converting to hydraulic boost. That sounds interesting, I should do some googling
     
  5. Golden Helmet

    Golden Helmet Full Access Member

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    That's called the hydroboost conversion, here's a link to a how-to on how it's done. Finding the pedal and the power steering reservoir is not easy, but if you can, hydroboost is absolutely worth it.
     
  6. Slicknik

    Slicknik Registered User

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    There is an easier test ,

    Let your truck sit for a while and then hook up a vacuum gauge at the point of where cluster of vacuum hoses meet the 5 way fitting under the dash. Start the truck and if your truck takes more than 30 seconds to reach 20+ on the vacuum gauge your pump is no good.

    But you should also check/replace the hoses that go from the pump to the dirt ok not and from the booster to the fitting


    You should also test your booster at the same time since all you have to do is watch the gauge and press the pedal and hold it to see if your booster is any good (truck needs to be running for this test as well)
     
    Last edited: Apr 22, 2019
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  7. TahoeTom

    TahoeTom Supporting Member Supporting Member

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    You will have to reuse your pully, so need the tool to pull it off the old pump.
     
  8. lkrasner

    lkrasner Registered User

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    Pulled the vac line from the pump and sure enough almost no suck. Nothing turning the pump by hand either. Put a new pump on and she's all good to go. Real easy job with a power steering pump tool borrowed from autozone.
     
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  9. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    Did you turn the new pump by hand? If you did, you can feel a big difference. A bad pump will just spin the pulley without much resistance. With a good one, you can feel the pumping action. That's not a sure fire way to tell though. Sometimes you can still feel a pumping action with a bad pump.
     
  10. lkrasner

    lkrasner Registered User

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    Yep. Felt next to nothing on the old one. New was much harder to turn by hand, impossible without the pulley on and immediately made vacuum with 1 or 2 turns.

    For the record, I was able to fit the extractor / instalation tool without removing anything. Kind of had to reach inside the fan shroud and hold the blades in the right place, but it wasn't too bad. If I could find my 10mm end wrench, probably could have removed the pump first then done it on the bench, but I just got the pulley off and took the bolts out with a socket. Belt went back on much easier after it had a few days to stretch out. I found it easiest to route around everything but the big idler pulley to the right of the vac pump, and slip it on that while holding the tensioner with a proper borrowed tool (way easier than fitting a breaker bar in there.)
     
  11. lkrasner

    lkrasner Registered User

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    It was only $5 of core charge, so I'm going to disassemble the old pump and see if I can tell what failed / attempt to fix it for the greater good.
     
  12. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    Without knowing much about these, I'd have to guess that the diaphragm failed. It would be AWESOME it we could repair these instead of buying another one.
     
  13. lkrasner

    lkrasner Registered User

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    It seems like they should be serviceable..the bearings on it still felt great. Seemed like a shame to spend $100 (and I got a good deal) on a whole new unit.
     
  14. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    I agree completely. I don't think that the drive part has much issues. It's mechanical and could fail, but that bridge could be crossed at that time.
     
  15. lkrasner

    lkrasner Registered User

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    I've heard the bearings do fail, especially from running a slightly short belt that puts too much force on it. But it's a standard sealed bearing held in with a snap ring, and easily replacec.
     
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