Disc vs. Drum Master Cylinder Bore Size?

Discussion in 'General Maverick/Comet' started by silver70, Jan 27, 2020.

  1. silver70

    silver70 Eric

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    Hey guys. It's been a while since I posted. Trying to get back into the Mav again. So, my '70 came with manual drums, all the way around, from the factory. I swapped in manual discs from a 4-door '74, but didn't take the master cylinder because, at the time, I was under the impression the drum brake master cylinder would work just fine. It worked, but was fairly hard to press. Is the bore of the manual disc master cylinder smaller to make it easier to actuate the brakes, or are both master cylinder bores the same?

    Thanks,
    Eric
     
  2. rthomas771

    rthomas771 Member

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    The bore size should be 15/16" on disc brake master cylinder. The '70 came with a 1" master. 1/16" shouldn't make that much difference as far as pushing the brake pedal with your foot.
     
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  3. silver70

    silver70 Eric

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    I wouldn't have thought so either, but after the swap I was like, "It seems unreasonable that Ford cleared a vehicle for production with this much braking effort required." It wasn't absurd, but it would have been a challenge for certain drivers. I just want it to be as correct as it can be. Thanks!
     
  4. Krazy Comet

    Krazy Comet Tom

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    The condition and correctness of proportioning valve can make a difference.

    I have the manual disc 15/16" MC on my Comet with '69 Torino discs, pedal is rather firm. After some workout(bedding pads), they are better but still nothing like power. Pedal stiffness is approx same as the manual drums on my Fairlane, but stops far better. Considering power conversion this spring. Power discs are in the Fairlane's future.
     
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  5. m in sc

    m in sc Member

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    year ago i worked at a place that built military vehicles, from scratch. we had to size the master as well on those units, you'd be surprised how much 1/16" difference can make a pedal feel.
    My galaxie has a leeds power disc conversion, with 4 piston calipers, and i'm going to swap the master (the one the kit came with) down one size on that for the exact same reason krazy comet mentions. It works well but the pedal feel isn't progressive enough, my 62 falcon with manual drums actually has better pedal feel. :/
     
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  6. dyent

    dyent Member

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    Smaller bore, easier pedal effort, but more travel, so pedal closer to floor. Larger bore, harder pedal effort, but less travel, so pedal is higher.
    David
     
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  7. silver70

    silver70 Eric

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    Thanks all. I took the manual disc prop valve from the '74 when I grabbed the spindles. My original manual drums had much easier braking effort. I'll get the 15/16" m.c. and try to track down a disc brake pedal on a pivot shaft for an automatic-equipped car. Mine was 3-on-the-tree originally and now has a c-4.
     
  8. m in sc

    m in sc Member

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    in the motorcycle world, when the master is too big, but it works but stiff, we call it 'wooden' feeling . Yeah, its a delicate balance to get it right. as long as you have a good amount room under your pedal now, the ratio can probably be played with a bit. But dyent is absolutely correct.
     
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  9. Krazy Comet

    Krazy Comet Tom

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    I looked for master cylinders with a smaller bore, Chevette has 3/4".

    I was fired up to try one but found mounting bolt holes are staggered vs in line.

    There have been good reports of using the single chamber boosters(I believe often referred to as hot rod boosters). Firewall requires some mods but with it flat against wall, there is room for it and MC.
     
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  10. m in sc

    m in sc Member

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  11. silver70

    silver70 Eric

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    I had consider a booster briefly, but I have everything on the car "gutted" to try to save weight. No PS, PB, A/C, heat, etc. I smoothed the firewall, as well, after removing everything. So unfortunately, no booster this time around.

    I want to try to keep most "regular service" chassis items factory if I can, but I appreciate the suggestion. It would probably make braking very easy.
     
  12. Krazy Comet

    Krazy Comet Tom

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    I have no idea if that Summit cylinder would be better but since the outlets are on inside, would at least necessitate modifying the hard lines(and most likely fittings). I didn't see anywhere mentioned drum vs disc.

    Pedal rod is also a consideration, the Raybestos MC on my Fairlane came with two different lengths, size could be matched to orig. Otherwise they were identical.
     
  13. m in sc

    m in sc Member

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  14. TeeEl

    TeeEl Member

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    Pedal effort will be high regardless of which master cylinder you use if you don't have power brakes. '74 was the first year discs became an option, and did not use a prop valve. It's the same junction block that the drum brakes had.
    '75 started using a prop valve for discs...
     
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  15. m in sc

    m in sc Member

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    i disagree. a smaller master will always reduce required pedal pressure at the cost of increased pedal travel. and vise versa.
     
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