How do I tell if slack adjusters are bad? Braking issues.

Discussion in 'Trucks [ Eighteen Wheelers ]' started by freightlinerman, Sep 1, 2012.

  1. freightlinerman

    freightlinerman Road Train Member

    Mar 4, 2011
    I have a 91 Ford L9000, single axle day cab with rear spring suspension. During hard braking, the front left wheel and right rear wheel are locking easily.

    I had the rear brakes adjusted by a mechanic, they said the right rear was over adjusted. They re-adjusted both sides to even them out. The right rear is still locking up easily, while it takes more effort for the left side to lock. They said the right rear slack adjuster needs replacement, since the drums and shoes are practically new. How do they know the left rear slack adjuster isn't bad? I was going to replace them as a set, they recommended replacing just one.

    They didn't measure the stroke of the chambers either, I'm no mechanic but that would seem logical, if the chamber on one side is pushing out more then obviously more force is being applied.

    The truck will attempt to swing out under hard braking. Normal braking, its fine. I only bobtail with this, I don't haul anything. They said one slack adjuster looks new than the other. Truck has 774k miles, I don't know the history.
  2. allan5oh

    allan5oh Road Train Member

    Jan 6, 2010
    Winnipeg, mb
    Are these automatic slack adjusters? If so there is a procedure (depending on mfg) to checking if they work correctly. I've found overwhelmingly when a brake is not adjusting correctly, that it is an issue with the foundation brake. I've seen broken/misplaced springs and rollers really screw up the actuation of the brake. Also if the s-cams or s-cam bushings are worn no amount of adjusting or changing the slack adjsuter will help. Usually brakes locking up is s-cam or s-cam bushings. That's fairly easy to check, with no brake pressure put a pry bar on the slack adjuster. If there's any significant movement up down or back and forth (some in and out is fine) then everything needs to come apart.
    Fatboy42 and freightlinerman Thank this.
  3. freightlinerman

    freightlinerman Road Train Member

    Mar 4, 2011
    They are automatic slack adjusters. What to the scam bushing do? I saw a video on you tube showing a worn out S-cam not applying the right pressure. What about that bushing? What role does it play?
  4. Gear head

    Gear head Medium Load Member

    Apr 1, 2012
    The s-cam bushings keep s-cam from flopping around in housing. If they are bad that allows s-cam to move which causes uneven brake pressure. Also if the bushings are bad the slack agjuster wont self adjust. What type of slacks are they. Do they get adjusted with seven sixteenths or does it have the squre head on top of slack. If it doesnt have sqare head then its a gunite or haladex. When u go to back this type off if they dont click or have resistance there bad. Yes you should measure stroke to see if brakes out of adjustment. If one wheel keeps having more sroke than rest then theres a problem.
  5. bender

    bender Road Train Member

    Jul 20, 2010
    Don't Kid Yourself
    You also want to take a close look at the pushrod clevis and pin to the slack adjuster connection. This connection can be froze up or worn out showing excessive play, elongated holes and worn out or missing bushings in the slack adjuster clevis pin hole. All these will cause the auto slack adjuster to not maintain proper adjustment.
  6. thesvg

    thesvg Light Load Member

    Dec 9, 2010
    This was an issue on my dads pete, after flat spotting four new tires on two different occasions, I tore into the brake drum only to find out a bunch of road sludge/ dry asphalt on the S cam lobes. That gunk was causing the brakes to lock up and wouldnt release when the parking brake was applied.
  7. freightlinerman

    freightlinerman Road Train Member

    Mar 4, 2011
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