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Increased master cylinder bore -- good or bad?

Discussion in 'The Garage' started by dremu, Aug 1, 2004.

  1. dremu

    dremu Officious Thread Derailer Premium Member

    Feb 27, 2000
    Likes Received:
    East of San Francisco
    Reading up on master cylinder swaps, as the brakes on my 3/4 /forums/images/graemlins/crewcab.gif need some love.

    Looks like in general, 1/2 ton's have a 1 1/8" bore, 3/4's get 1 1/4" and 1 tons get 1 5/16".

    So, the question is ... is a larger bore better for braking? Seems to me like it would be, all other things being equal, as it would move more fluid. However, one web page I found about it mentioned that while the volume INCREASES, the pressure DECREASES with the bigger bore.

    And I can't decide whether that's good or not. Any ideas?

    If the bigger bore is overall a good thing, I'm gonna go to the 1-ton on my /forums/images/graemlins/k5.gif as well as my /forums/images/graemlins/crewcab.gif.

    One thing that confuses me is that when I replaced the master on my '77 /forums/images/graemlins/k5.gif with one from an '85 /forums/images/graemlins/k5.gif (i.e. still 1/2 ton) the bore was much larger... I swear that K5 had the big brakes already, but I'd also swear that the bore was much less than an inch. Any ideas?

    Small print: Yes, I know I gotta switch boosters at the same time, and maybe switch lines front-to-back.

    Any and all opinions, facts, part numbers, disagreements, and general mayhem welcomed.

    -- A
  2. ntsqd

    ntsqd 1/2 ton status

    Nov 28, 2002
    Likes Received:
    So. CA
    It's probably not a good thing unless the other related components from the donor application are also put on the truck. Brake systems are a complicated balance of bore sizes, boosters, and leverage ratios. Too much or too little with some part can easily throw off the balance.

    A bigger m/c bore will move more fluid. This is good if the pedal travel is too long. It will also DECREASE the total leverage in the system which means you will have a firmer, but less effective brake pedal. So it possibly could feel really good, and stop like shiat. Unless you offset this with more boost from the power booster you'll probably hate the change.

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