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Thread: M939 Front Brake adjustment?

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    Default M939 Front Brake adjustment?

    I am working on a 1982 M939 that the PO said the front brakes don't work that well and need adjusted. I pulled a front drum off and didn't see any adjustments, the drum and shoes are in great shape with plenty of material on them. Is there an adjustment for the front brakes or are they self adjusting?

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    Self adjusting wedges. Ensure your wedge is kicking out like they are suppose to. They can rust and get gummed up and need lubrication to get them to move their full range of motion. You can remove the drum and apply the brakes and see what movement you have. May pull the wedge apart. Pull it apart. Clean it grease it and reinstall
    2008 F-250 6.4L Powerstroke
    1984 AM General M923A1 ( Big Cam III NTC-400, HT4B turbo, Intercooled, #10 button, Dual fuel Line setup - MRAP Aluminum Radiator - 7" Ram Air Intake system - Dual 115 gallon tanks - Air Conditioning - 145amp alternator - Puradyn Oil Bypass system, CAT 7155 16speed transmission, Oshkosh 55000 Transfer Case, riding on 16.00 AT-3s on HEMTT wheels. )
    M923A1
    M931A2
    M1083
    M1078
    M796 Bolster
    M322 Equipment Trailer
    MEP-804A


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    I pulled the drum off and had someone push the brakes and the drums didn't move.

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    The shoes didn't move? Ensure you have air going to the chamber on the lines in. Are you having any air loss on the tanks with the brakes applied? You could have a blown brake diaphragm which will not allow them to apply. If you remove the line on the end of the can side and use an air gun or rig up a fitting and apply air the brake should engage unless the diaphragm is bad. If it's good then check the actuator rod in the chamber. If it's good remove the wedge assembly. They have a rebuild kit for them. Probably just needs lubrication though
    2008 F-250 6.4L Powerstroke
    1984 AM General M923A1 ( Big Cam III NTC-400, HT4B turbo, Intercooled, #10 button, Dual fuel Line setup - MRAP Aluminum Radiator - 7" Ram Air Intake system - Dual 115 gallon tanks - Air Conditioning - 145amp alternator - Puradyn Oil Bypass system, CAT 7155 16speed transmission, Oshkosh 55000 Transfer Case, riding on 16.00 AT-3s on HEMTT wheels. )
    M923A1
    M931A2
    M1083
    M1078
    M796 Bolster
    M322 Equipment Trailer
    MEP-804A


    901-318-8353 for Help in and or around the Memphis area or transporting in the southeast. Text me first before calling.

    Tires (HMMWV, 395s, 1600s) and Transportation Vendor

    Wes

    Videos at https://www.youtube.com/results?search_query=5782simp

    https://www.youtube.com/channel/UCTd...D4dOJidPFnmYGA

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    Yes the shoes didn't move not the drums, so which line do I apply air to? I have never worked with air brakes before.

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    The one on the end cap. The tube that controls the wedge has 2 lines and a rubber diaphragm. the one on the frame side applies air. It forces the rubber diaphragm in and actuated the wedge and brake. The other line on the inside vents the air when released.
    2008 F-250 6.4L Powerstroke
    1984 AM General M923A1 ( Big Cam III NTC-400, HT4B turbo, Intercooled, #10 button, Dual fuel Line setup - MRAP Aluminum Radiator - 7" Ram Air Intake system - Dual 115 gallon tanks - Air Conditioning - 145amp alternator - Puradyn Oil Bypass system, CAT 7155 16speed transmission, Oshkosh 55000 Transfer Case, riding on 16.00 AT-3s on HEMTT wheels. )
    M923A1
    M931A2
    M1083
    M1078
    M796 Bolster
    M322 Equipment Trailer
    MEP-804A


    901-318-8353 for Help in and or around the Memphis area or transporting in the southeast. Text me first before calling.

    Tires (HMMWV, 395s, 1600s) and Transportation Vendor

    Wes

    Videos at https://www.youtube.com/results?search_query=5782simp

    https://www.youtube.com/channel/UCTd...D4dOJidPFnmYGA

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    I have never worked on military wedge brakes . I have worked on regular wedge brakes in trucks and I remember using a brake spoon or screw driver to adjust the shoes through the oval holes in the backing plates and turning the star wheel that moves the plunger out to move the shoe out closer to the drum

    Some times the plunger could be stuck from mud and time or the star wheel could have the stars broken off from trying to be forced because its stuck .

    The wedge brake works by the push rod from the air chamber pushes the wedge between the plungers forcing shoes a part and when they release they should self adjust by turning part the plunger assembly with the spirals on it that should turn the star wheels to make them self adjust . I'm thinking there is a small spring loaded locking tab to lock the spiral part .

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    Rear brake setup. Front is the same it only pivots on one end though
    Attached Images Attached Images
    2008 F-250 6.4L Powerstroke
    1984 AM General M923A1 ( Big Cam III NTC-400, HT4B turbo, Intercooled, #10 button, Dual fuel Line setup - MRAP Aluminum Radiator - 7" Ram Air Intake system - Dual 115 gallon tanks - Air Conditioning - 145amp alternator - Puradyn Oil Bypass system, CAT 7155 16speed transmission, Oshkosh 55000 Transfer Case, riding on 16.00 AT-3s on HEMTT wheels. )
    M923A1
    M931A2
    M1083
    M1078
    M796 Bolster
    M322 Equipment Trailer
    MEP-804A


    901-318-8353 for Help in and or around the Memphis area or transporting in the southeast. Text me first before calling.

    Tires (HMMWV, 395s, 1600s) and Transportation Vendor

    Wes

    Videos at https://www.youtube.com/results?search_query=5782simp

    https://www.youtube.com/channel/UCTd...D4dOJidPFnmYGA

  12. #9
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    I rigged up a fitting so I could apply air to the actuator, and the wedge and shoes moved freely when I applied air. I believe that means I have an air supply issue.

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    4 Star General Jbulach's Avatar
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    Quote Originally Posted by 00t444e View Post
    I pulled the drum off and had someone push the brakes and the drums didn't move.
    Make sure your helper wasn’t pussy footing the pedal, these trucks brakes are proportioned very heavy to the rear.
    M925A2

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