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Brake Booster Rod adjustment

Discussion in '365 GT4 2+2/400/412' started by Tojo, Jan 4, 2022.

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  1. Tojo

    Tojo Formula Junior

    Apr 12, 2002
    497
    Sydney
    Full Name:
    Tim
    #1 Tojo, Jan 4, 2022
    This is the rod that goes inside the brake booster and pushes against the master cylinder. It’s an adjustable length. Does anyone know what that length is meant to be? Mine has come from another unknown car, so I can’t be certain it’s correct for the 400, and I found no info in the WSM, or I’ve looked in the wrong place. Currently it’s 120mm in length. Thanks
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  2. Arvid

    Arvid Formula Junior

    May 28, 2012
    668
    Norway
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    Arvid Andersson
    #2 Arvid, Jan 7, 2022
    Don't have any fixed length to recommend but I've done this a few times on my cars and I try to adjust it to 1mm clearance from the pushrod tip to master cylinder piston. I suspect if you set it to a fixed length it would differ if you were to replace the master cylinder or booster and that it needs to be adjusted individually when replacing components.
     
  3. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,512
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #3 Steve Magnusson, Jan 8, 2022
    Last edited: Jan 8, 2022
    I don't have a copy of the 400/400i WSM, but, going thru the WSMs that I have, they went more to specifying the brake pedal free play (and often say "replace both as a unit" ;)). The last one I can find where they did give a dimension for the clearance at the rods is in the Mondial8/QV WSM. Some examples:

    308GT4 WSM -- Drawing shows a clearance and gives a dimension range. Brake pedal freeplay after adjustment is specified in the text:
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    Mondial 8/QV WSM --Drawing shows a clearance and gives a dimension range. Brake pedal freeplay after adjustment is specified in the drawing and in the text:
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    TR WSM -- Drawing shows a clearance, but no dimension given -- brake pedal freeplay spec in the text and a (different) drawing but on a different page:
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    512TR - Drawing shows a clearance, but no dimension given -- brake pedal freeplay spec shown in drawing on a different page and in text on yet another different page:
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    Anyway, just wanted to mention that you can use the brake pedal freeplay as another sanity check for the adjustment, and, if given in the 400/400i WSM, it might be rather somewhat hidden in the text. I don't mind Arvid's suggestion maybe being on the side of giving extra clearance as what you definitely don't want is less than zero clearance.

    PS I'll give the 512TR WSM an award for one of the worst English technical translations ever for "sucking expansion box" ;)
     
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  4. Tojo

    Tojo Formula Junior

    Apr 12, 2002
    497
    Sydney
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    Tim
    #4 Tojo, Jan 8, 2022
    Sucking expansion box :D:D That's gonna give a few laughs with the boys at work!!

    Thanks for the info, it's pretty helpful. I haven't played much with boosters and master cylinders, so I assumed the rod was a set length. More research learned me it's the clearance between the rod, and master cylinder piston that is needed. There's some good videos on youtube about measuring and calculating the clearance, also there's a tool you can buy as well https://www.amazon.com/Power-Brake-Booster-Adjustment-Tool/dp/B079QGXY55. Most forums and videos seem to recommend 0.5mm or 0.20" as the clearance. I think I'll aim for the 0.3mm from the drawing you posted before
     
  5. Motob

    Motob Formula 3
    Professional Ferrari Technician

    Nov 11, 2003
    2,342
    Frederick, Maryland
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    Brian Brown
    #5 Motob, Jan 8, 2022
    The 0.3mm is a good value to shoot for. You want to run as close to zero as is possible in order to have the least brake pedal travel.
    What you are trying to avoid is having too little clearance or negative clearance, which can result in the front brakes sticking on, as you will have some changes in the dimensions of the rod and master cylinder piston due to high under hood temperatures. This can cause the piston in the brake master to cover up the compensation port, keeping pressure on the front brakes.
     
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  6. Motob

    Motob Formula 3
    Professional Ferrari Technician

    Nov 11, 2003
    2,342
    Frederick, Maryland
    Full Name:
    Brian Brown
    #6 Motob, Jan 8, 2022
    One check that I do is to look into the rear port on the brake master cylinder while I am tightening down the master to the booster fasteners. If I see the rear piston move relative to the port, then I know that I have negative clearance, and have to readjust the rod shorter.
     
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