Adjusting Brake Booster

Discussion in 'General Motoring' started by Derek Lawler, May 22, 2006.

  1. Derek Lawler

    Derek Lawler Guest

    After tediously getting the brake booster out of my 1986 Honda Civic sedan
    (AT just rebuilt, 1.5L engine) and replaced with a rebuilt one I could not
    get it through the firewall without removing the 2" clevis connector so I
    had to back out the replacement far enough to put it on from the inside and
    then put the pin and cotter pin in place. Bled the rebuilt master cylinder
    on the bench then bled all four wheel cylinders. However, the pedal goes to
    the floor and since I can't screw the clevis further back on the booster's
    threaded rod I am trying to back off the rod from the booster where there's
    a locknut. Using pliers I managed to back it out a fraction of an inch. Is
    this the right way to go about it? I would appreciate any input on this. I
    do have braking but the pedal goes too far down for my peace of mind.
    Derek Lawler, May 22, 2006
  2. Derek Lawler

    Jim Yanik Guest has a procedure for adjusting the brake booster.
    You have to make some special tools by grinding down standard wrenches.
    Jim Yanik, May 22, 2006
  3. Derek Lawler

    Derek Lawler Guest

    Thanks for the link, Jim. Will turning the nut on the booster damage it?
    What goes on inside the booster?
    Derek Lawler, May 22, 2006
  4. Derek Lawler

    Jim Yanik Guest

    I don't think so;

    IIRC,there's an adjusting nut and a locknut to keep it from moving-those
    are what you have to adjust,and why you need the modified wrenches;they are
    12 point nuts,like the inside of a 12 pt.wrench.You adjust the freeplay
    between the booster pushrod and the brake master cylinder's piston.

    AFAIK,the booster uses engine vacuum to assist the pedal force on the rod
    that pushes the brake master cylinder.There's a rubber hose from the
    booster to the engine's intake manifold to supply vacuum.
    As far as what the internals are doing,I don't know for certain.I imagine
    there's some sort of opening valve to modulate how much vacuum gets applied
    to the diaphragm that adds force to the pushrod.
    Jim Yanik, May 22, 2006
  5. Derek Lawler

    TeGGeR® Guest

    What's your pedal freeplay like? Should be less than 5mm.
    TeGGeR®, May 23, 2006
  6. Derek Lawler

    Derek Lawler Guest

    I don't want to go to the trouble of pulling the booster back enough to run
    the clevis back further, since I would have to take off the brake lines from
    the MC and those nuts on the booster studs are impossible to get
    to ---especially the right top one--I used a piece of tubing on a 12mm
    socket to get the threads started and then tightened using a universal joint
    on an extension. Maybe I could hire a midget to do this for me next time.
    I also have not ground down a couple of comb wrenches to fit over the
    adjustment nuts. Would line wrenches work on this---I haven't tried? If I
    did that and screwed the rod further in how would I be able to tell about
    the clearance? The diagrams are good as are the photos but after looking at
    it over and over I still am not clear about the MC and pedal freeplay. I am
    ready to turn it over to someone who has done this before and pay them for
    I have brakes and could live with it this way I suppose---pedal free play
    "feels" like 5mm.
    The engine stalls on me as I roll to a stop as if one of the plug wires is
    off (not the case) but I wonder if the guys that did the tranny didn't make
    the throttle adjustment as it shows in the book. Could the valve in the
    vacuum hose be a problem? I went to buy a new one at the dealer and they
    want fifty bucks for that little nylon valve! This car is old enough that
    most younger mechanics probably haven't worked on one so I am reluctant to
    turn it over to anyone yet.

    Derek--quite puzzled, but thanks very much for your input.
    Derek Lawler, May 24, 2006
  7. Derek Lawler

    TeGGeR® Guest

    You don't do that. You adjust by turning the threaded pushrod itself in or
    out until correct freeplay is achieved. Study the text and photos, then
    study your own car's arrangement.

    You'll surely find they will be too thick and the wrong angle. As you've
    already seen, space is stupidly tight under there.

    When you push the brake pedal with your fingers, push *VERY* lightly. An
    extremely delicate touch is needed here. Freeplay will be VERY light
    compared to the effort needed thereafter.
    See here for why there's freeplay in the first place:
    See the second graphic.

    It ought to be quite obvious. If you feel no lightness at all at the top of
    pedal travel, possibly freeplay is currently zero, which can be a bit of a

    The valve is just a one-way valve. Pull it off the booster hose. Now blow
    one way through it, then the other way. There should be no flow one way,
    but flow in the other direction. These normally don't go bad. And even if
    they do, you'll find scads of them for dirt cheap at the wreckers. AFAIK,
    most Hondas use the exact same valve.

    How tightly are the REAR brakes adjusted? If they're too loose, you'll get
    long pedal travel, even if the parking brake seems OK.
    TeGGeR®, May 24, 2006
  8. Derek Lawler

    Derek Lawler Guest

    Thanks again for clarifying this procedure. You are one who can "suffer
    fools gladly." I appreciate your patient explanations. I must commend you
    on the site you have set up for us Honda lovers with explicit diagrams. Now
    I will go and grind a couple of cheap tools to fit. I will also check the
    adjustment of the back brakes as you recommend.
    Derek Lawler, May 24, 2006
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