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De-linking SP brakes

Discussion in 'Loop & Tonti' started by T Bill, Mar 25, 2017.

  1. T Bill

    T Bill Tuned and Synch'ed GT Contributor

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    I was just buying brake caliper and m/c rebuild parts for my '83 SP NT, and got to thinking I'd always wished the brakes weren't linked. Makes me really uncomfortable when I find myself on dirt roads. Maybe I should just do it if I'm gonna get my fingers in the fluid. I would prefer not to argue this point, everybody has an opinion.

    I see I can get a 15mm master cyl for the front for less than $150, but don't see any rear masters available. I'm guessing I'd want a 12mm for that, going by some old threads here. Anybody know where to find them?

    I have the 15mm and dual disc front setup on my 850T, and have (very briefly!) locked the front wheel on that with two fingers. Like the feel of that. Would leaving the stock 15mm rear master cylinder in place on the SP be OK? It's a foot brake, so maybe the feel isn't such a big deal? Anybody already running this setup?
    Thanks,
    Bill
     
  2. Amboman

    Amboman Cruisin' Guzzisti

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    The rear on older Tontis (such as my '76 Convert) are 15 mm. I saw a post on Wildguzzi(?) where someone used the front (cable operated, under-tank) master cylinder from a V50 II for the rear, with excellent results.
     
  3. T Bill

    T Bill Tuned and Synch'ed GT Contributor

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    Thanks guys, I think I'll move ahead with this. I had sort of forgotten about that ratio valve or whatever it is that splits the feed to the front and rear calipers. Can I just eliminate that and send the rear master line straight to the caliper? This should end up simplifying the plumbing a bit.
     
  4. Stevex

    Stevex Tuned and Synch'ed

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    I've de linked my LM2; like you are thinking, I put a PS15 on the front.
    I fitted a PS12 on the rear; it came from a Ducati 860 GT (NOS from your side of the pond), a mirror image of the original Guzzi rear master that just requires a little machining to allow it to fit the 'right' way round. You can then bin the proportioner valve.
    These pics shows the PS12 after the required machining, not much at all.
    I think the original Guzzi PS15 feeding the single rear caliper will be quite fierce and lacking any feel.

    [​IMG]

    [​IMG]
     
  5. T Bill

    T Bill Tuned and Synch'ed GT Contributor

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    Plan for now is to try the existing rear MC as-is. If it turns out I don't like it, where did you find your Duc unit on this side of pond?
    Thanks,
    Bill
     
  6. jr1967

    jr1967 Cruisin' Guzzisti

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    I've done that in the past(with the 15mm original rear); there is no feel and no bite, you can stand on the rear brake with your whole weight for some braking but you get the idea. No good. Tried it with a 14mm master that I had laying around. Still no good.
    The V50 master would be the way to go.
     
  7. T Bill

    T Bill Tuned and Synch'ed GT Contributor

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    jr1967, is it possible you left the proportioning valve in the line, further reducing pressure to the rear? The ratio of the piston areas says one would get 35% less pressure at the caliper for a given force on the pedal with the 15mm master. Seems like that wouldn't be enough to produce the effect you describe.
     
  8. jr1967

    jr1967 Cruisin' Guzzisti

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  9. Stevex

    Stevex Tuned and Synch'ed

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    I got it from ebay, but my purchase records don't go back far enough to i.d. the seller, sorry.
     
  10. T Bill

    T Bill Tuned and Synch'ed GT Contributor

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    Followup report:
    Mods are done and the pads have had a couple hundred miles to wear in. New hoses I had made at Pirtek laid in perfectly (whew!). Both front and rear brakes work great. Turns out the rear caliper is an F09, with 48mm pistons, which I'm sure helps a lot with the 15mm stock rear master cylinder. I can lock the rear wheel with firm pressure on the pedal, don't think I'd want it any more sensitive than it is.
    Happy camper here.
    Bill
     
  11. T Bill

    T Bill Tuned and Synch'ed GT Contributor

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    Actually just consulted the chart jr1967 suggested, and see that my front ratio is 25.67, rear is 20.48, values which fall pretty well into the suggested ranges. 20.5 sounds a little low, but the rear is operated by foot, through a linkage that may be giving some mechanical advantage of it's own. Maybe that's why this hydraulic ratio seems to work pretty well.
     

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