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Thread: D4-7U hydraulics will not hold blade up slowly drops within a minute or two

  1. #1
    Join Date
    May 2019
    Location
    Mountain House California
    Posts
    22

    Default D4-7U hydraulics will not hold blade up slowly drops within a minute or two

    Hello,

    The hydraulics will not hold the blade up on the D4 it slowly drops over a minute or two. I do not see any hydraulic leaks and the hydraulics operate smoothly and can lift the dozer but it had been sitting for 15 years. I see what looks to be a manual bypass valve and its set to centered, and there are two quick connections in the back of the CAT. Should this valve be centered and would it cause this? I haven't attempted to move the valve yet and wanted to hear what others think first.

  2. #2
    Join Date
    Nov 2017
    Posts
    446

    Default

    do you have one lever or two?
    D4 6U,1943 D4 2T,Laplant Choate C41 Carrimor Scraper

  3. #3
    Join Date
    Nov 2006
    Location
    Corralitos, Ca.
    Posts
    14,746

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    Diverter valve handle should point to the right for dozer operation, straight back for rear implement operation.
    Blown hydraulic ram piston seals are most likely cause of leak down.
    Get a part number off the cylinders.

  4. #4
    Join Date
    May 2019
    Location
    Mountain House California
    Posts
    22

    Default

    Only has one lever, I'll move it to the right and get the part numbers as suggested. Should I try that Lucas hydraulic booster with stop leak?

  5. #5
    Join Date
    Nov 2017
    Posts
    446

    Default

    one lever. then OM suggestion would be right on. do the packings leak around the polish rods?
    D4 6U,1943 D4 2T,Laplant Choate C41 Carrimor Scraper

  6. #6
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    May 2019
    Location
    Mountain House California
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    Well, I did not see any visible leaks after about 30 minutes of use and when raising or lowering it operates smoothly.

  7. #7
    Join Date
    Dec 2006
    Location
    NE OKLA
    Posts
    5

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    My 6U was bypassing the piston seal, had the same leak down. No outside leaks. Rebuilt the cylinders and everything is great

  8. #8
    Join Date
    Feb 2009
    Location
    Vining, Minnesota
    Posts
    486

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    As old magnet says most likely cause will be an internal leak around the piston seals. I don't know what medium was used for those seals. If rubber a hydraulic stop leak may work. How that stuff works is it "attacks" the rubber causing it to swell. At best it's a temporary stopgap measure. Mechanic in a bottle stopgap measures may work for a time but are not going to last like a proper repair. They may also cause excessive wear or destruction of parts in the system.

    Rick

  9. #9
    Join Date
    May 2019
    Location
    Mountain House California
    Posts
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    Sounds good, I'll start researching on how and what's all involved to remove and replace the bad piston seals.

  10. #10
    Join Date
    Nov 2006
    Location
    McBEE, SC
    Posts
    2,335

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    There are at least two different kinds of seals, one of them are composite rings with a stepped end gap. It is possible to use a modern piston seal and make a new groove in the middle of the piston if needed.
    D2 5u & 3J
    212 9T
    RD4
    D4 7J & 7U
    R4 4G
    Gas 35's
    Diesel 35
    Diesel 40
    Gas 50's 5A
    Diesel 50 1E1755
    Diesel 75
    D 8800 power unit 9J 6113SP
    LeTourneau D4 tournapull/Q Carryall
    Letourneau D Carryall's
    LaPlant-Choate CAB-97

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