Author Topic: New SS brake lines can make squeezing the brakes like gripping a stone. New MC?  (Read 1077 times)

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Offline Autoground

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Hey guys,

Once upon a time, an extremely knowledgable mechanic helped me install SS brake lines on an old Yamaha. He wisely knew that with the lines I needed an alternative MC. With the new MC, the brake lever distance travel was longer and smoother, with more precision in grip.

I am now without the aid of this wizened guy. I have a new SS line for my cb650c.

In you guys' experience, when installing SS lines, is a MC conversion also useful for this bike to get back lever precision? And if so, what MC is best to order?

Thanks so much
1980 CB650c

Offline slikwilli420

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I've never heard of changing master cylinders because you went to stainless lines. There isn't any logic in that I can see.
All you gotta do is do what you gotta do.

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Offline scottly

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He wisely knew that with the lines I needed an alternative MC. With the new MC, the brake lever distance travel was longer and smoother, with more precision in grip.

Like slikwilli said, the lines have nothing to do with the MC size. On your Yamaha, the mechanic fitted an MC with a smaller piston, which gave more "leverage", and the longer travel at the lever. Honda was notorious for using too large of MCs; the 14mm masters used on all single disc Hondas worked much better with dual discs. On the factory dual disc bikes, it was worse, as they used a huge MC, like 17mm? What year is your 650? There was a change from single piston calipers to two piston calipers in 1982, I think. Are there any markings on the master, like 11/16?
Don't fix it if it ain't broke!
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Offline Autoground

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Yes, Scottly, thank you! I remember now that was just what he described-- the piston bringing leverage.

Mine is a 1980 and I know it's a single pistoned caliper as I have is dismantled for cleaning right now. The only markings are on the underside and one of them has a funny symbol:

4. S. 1. *14.

Is single piston worse for my scenario? Is a smaller pistoned MC an easy find? Ha.

Or is this brake handling just what everyone lives with and that's ok?

1980 CB650c

Offline Autoground

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Thanks to the info in another thread I know I'm on the lookout for a 12mm MC. Taking suggestions in case anyone feels strongly about a particular option
1980 CB650c

Offline Rosinante

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A fatter MC piston will increase the pressure needed for a given amount of braking, and vice versa.  It looks like someone is suggesting that perhaps a fatter MC piston is needed for SS lines?  Because the SS lines will reduce the sponginess of the system?

Generally speaking, and certainly for performance automobiles, drivers prefer to reduce sponginess as much as possible.  This is certainly the case with my performance car.  I want the pedal to be as FIRM as possible.  Once the brake pedal is engaged and stopping begins, the pedal does not travel any further.  Additional pressure gets additional braking power, without the pedal moving perceptibly AT ALL.

Candidly, I find the feel of my front brake (1978 CB750K) to be somewhat spongy.  Those brake lines should probably be replaced and I would not hesitate to upgrade to the more rigid (and likely less spongy) SS lines.  I would not expect to feel a need for MC redesign. 
1978 CB750K

Offline robvangulik

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A SMALLER MC piston will increase the pressure while using less handpower for any given system.

Offline Rosinante

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Yes, I think we said the same thing.  Some folks imagine that a fatter piston gives more braking power but the opposite is true.  A fatter piston increases the pressure needed to achieve a given braking power.  I am not a motorcycle race guy, and that may be different from cars.  Motorcycle handle bars move a bit, and it is possible that a highly sensitive and firm brake lever might be a bad thing.  I dunno.  In performance cars, the higher and tighter a brake pedal is, the better.  To a point, of course.
1978 CB750K

Offline Don R

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 What Scottly said.
 The smaller piston requires less lever pressure but needs more travel to do the same job. This week I noticed my 750 K0 lever is like gripping a stone. I've been used to a stock gl1000 m/c with dual piston V65 calipers. 
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Offline Autoground

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Yes, the smaller the MC piston, the sharper the stopping power (and the longer the lever travel). Apparently Honda deliberately oversized the bores to avoid any rider ever locking up the front wheels.

I've actually decided that I want to aim for an 11mm MC! Fortunately, there is a list of Hondas with 11mm MCs that can be found

https://www.google.com/url?q=https://www.caferacer.net/threads/needed-small-bore-master-cylinder-11mm.23424/&sa=D&source=hangouts&ust=1597981083530000&usg=AFQjCNGLLj9JpvS7ZA5OUfOW4mtwgFzh7g

So I'll be going with whatever newest MC I can find. :)
1980 CB650c

Offline scottly

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You may be able to swap a two piston caliper from an '82 or newer bike using factory parts; the V65 calipers Don mentioned have two 32mm pistons, which are the equivalent of a 45mm+ single piston. I'm using one of these calipers with a stock 14mm MC and a larger-than-stock 320mm Ducati rotor, and it works very well.  ;D
« Last Edit: August 19, 2020, 09:49:23 PM by scottly »
Don't fix it if it ain't broke!
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Offline dugsgms

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I switched from a 14mm to a 12mm on my hot rod RD350(very similar front brake) along with a SS line and it was a nice change. More stopping power and better modulation with very little change in travel. I used a generic a master from MikesXS with a built in brake light switch, happy with the results.
1970 BSA Lightning
1975 Yamaha RD350
1975 Honda CB400f
2008 Yamaha WR250R

Offline Autoground

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Me, after I did an excessive amount of searching for 11mm (I found several, a few with brake switches, and none with mirror screws), I moved on to hunting 12mm Honda pistons. I was unable to find a source/confirmation for any of them.

I settled on the Buell Blast that has been recommended from several sources. I am not sure if it is 12mm or 12.7 but it's the best option that I found.
1980 CB650c