Setting rear brake free play

Discussion in 'Trials' started by Ymirtrials, May 8, 2018.

  1. Ymirtrials

    Ymirtrials Been here awhile

    Joined:
    Jan 12, 2012
    Oddometer:
    267
    Location:
    Ymir BC Canada
    I could find anything relevant after a search here so new topic for an old subject I'm sure.....
    Anyhow, snapped off my brake pedal on Sunday so on goes the spare and in getting it set up I can never seem to tell where the free play ends (if there is any) and when the piston starts to travel. The biggest reason I'm looking for input here is that I try to set up my rear brake with the minimum travel possible for full application but in the past I have set it up too tight and roasted my brakes coming down the mountain after a long ride.
    Anyone have any words of wisdom here?
    #1
  2. motobene

    motobene Motoing for 46 years

    Joined:
    Mar 9, 2013
    Oddometer:
    4,789
    Location:
    Wichita Mountains SW Oklahoma
    Very simple.

    You can roast brakes by riding them too long, or by over tightening the push pin such that you move the piston off the home position snap ring such that it shuts off the master cylinder reservoir refresh hole. That happens inside of the first millimeter of master cylinder piston travel. Blunder the slack adjustment and the rear brake lever will go flaccid after the roasted brakes cool, as the fluid can't refresh.

    Remember, the brakes are open circuit from the caliper pucks to the reservoir cap bladder - or should be - with levers in the home, at rest, position.

    Set lever position ergonomics where you prefer, allowing room for the lever to not bottom when depressed to full lockup. Brake lockup should be the limit of the lower stop, not, say, the skid plate.

    The upper stop is where the lever can't be pulled up any more.

    You need to feel, minimally, a touch of slack at the push pin and master cylinder piston when you jiggle the lever with dainty fingers against the upper stop. You don't need much, but you must have a little felt slop.

    Adjusting the pin length via the 10mm hex and M6 thread of the push pin gets you there.

    BTW, the amount of displacement of the push pin is usually no more than 3/16" (4.7mm) from un depressed and resting against the piston and wheel lockup. If there is more you may have a bubble in the rear brake.

    Mark the un depressed and depressed positions on the push pin at the rubber bell seal of the master cylinder. Lever feel should be firm, not spongy, at about 1/8" (3mm) of push pin stroke under the force of aforementioned dainty fingers. Engaging the brake should feel like a nice solid bump.
    #2
    Norman Foley and jonnyc21 like this.
  3. Ymirtrials

    Ymirtrials Been here awhile

    Joined:
    Jan 12, 2012
    Oddometer:
    267
    Location:
    Ymir BC Canada
    Ok thanks for the reply, I’ve started by backing off both the push pin and the limit bolt to find where the pin just contacts the piston and set the return bolt there as it felt like a good position then with dainty tea todling fingers checked the travel and I’m bang on 3/16”. I may try to bleed from the caliper as I have not changed the fluid yet but this is about how much pedal travel I usually have. I feel like some bikes that I test out have less pedal movement, going to keep at this to get it as good as possible because I’m on the brake all the time now trying to improve my skills! :ricky
    #3
    motobene likes this.