Topic: Input about front master cylinder size for CB750 Dual disc modification !

[Viktor.J]

Hi !
For starters id like do apologize for my bad English, its not my mother tongue. 

I'm doing a dual disc modification on my CB750 K2.
And I'm searching for the new master cylinder. I found this master cylinder chart on the web vintagebrake.com/mastercylinder (Dont know how to get rid of the "you arent alowed to post external links" error message) and now I´m  trying to understand this. On the left hand i have the break piston size (it says piston, but isnt the piston size whats matters ? ) which in my case is 38 mm ? and on the top i have the master brake cylinder size.
 And since i have 2 calipers with each ONE active piston, i have 2 active pistons.
According to this persons recommendations I should use a master cylinder between 10-11 mm to reach a relation between 28:1-23:1 ?!?
 Isn't 10-11mm quite small ?

AND
Does anyone have a list or something to find and compare different master cylinder sizes ?
So i can find a master cylinder that fits the dual disc assembly.

Kindly regards
Viktor Johansson

[Viktor.J]

BUMP !


If I use this ratio, and i use the original brake piston size (38 mm right ? ) and i use dual disc which makes two active calipers. And he recomend a ratio between 20:1 or 30:1. the resault comes out as.

 => Atot,c=2268,2 mm^2
=> Amc,20:1=113,4 mm^2 och Amc,30:1=75,6 mm^2
=> Dmc,20:1=12 mm och Dmc,30:1=10 mm.
where c= caliper, mc=master cylinder.

But the only info ive found on the orignal MC size is 5/8" wich is about 15 mm ?? that would be to big ? ?? ?

what is wrong with this ?

Peace
Viktor

Då är det bara att ge sig ut på jakt för en huvudbroms cylinder med en storlek på  10<Dmc<12 mm.

[fastbroshi]

Viktor, you can use google translator and type in what you want to say in your native tongue and it will convert it over to English.  Ten or 11mm sounds about right for the master cylinder. 

[Viktor.J]

I know about Google translate but dont find it nessesary. Or is My english that bad ?

But if The original master cylinder size For cb750 K2 is 14mm, then shouldnt i use a bigger master cylinder when i do The dual disc convertion ? not smaller as this "brake piston to master cylinder ratio suggest (10-12mm) ??

This is how My thinking:
I mean, more calipers ( 2 insteed of 1 active) needs higher brakefluid pressure to performe correct and effective. This increase causes by bigger master cylinder ! 
Is this Reasoning right ?

// Viktor

[Viktor.J]

Thanks, it feels good to be understood

But this ratio chart then, isnt it accirate ? Cause these calculations is way out
If i were to use The original 14 mm mc i would get a ratio of 14:1 which would be verry "low" and give a "wooden" feeling. But The Reasoning you change isnt that cause The main MC is to small = gives à "weak" and non-efficient feeling ?
This Contradict eachoter ....

And manu recomend The GL1000 MC which should be 20 mm and give me à ratio of 7:1 ..... Crazy #$%* hahah

Why al this contradiction ? Whos wrong ?


// Viktor

[Terry in Australia]

Hey Victor, unless you're really keen to keep that traditional MC look, install a MC from a later bike, I'm using a Suzuki 1200 Bandit MC on my bike, but any late 1980's - to present model MC's are just so much better. I've tried GL1000, CB750F2, CB900F MC's over the 34 years I've owned CB750's, but they were rubbish compared to modern MC's. Cheers, Terry.

[Viktor.J]

Yes thats what im planing to do but this "brake caliper area: MC area"-ratio chart is missleading !
Cause according to that you should go DOWN in MC size !

I guess this brake caliper area: MC area"-ratio chart is a big pile oh #$%* then ?

[Terry in Australia]

No, I think it's correct, from memory, modern MC pistons are smaller in dia, but have longer strokes. I didn't worry about all that crap, I just bought a modern MC and it works great. Cheers, Terry.

[bryanj]

When i did a 500 years ago the standard worked but had a long pull and felt spongey so i fitted an early GL1000 one which had the added advantage of the brakelight switch being fitted to the master cylinder as i had plumbed the second caliper to the original hydraulic switch hole.

I also do not understand the tables as to make two caliper pistons move must need more fluid than just one so for the same lever it has to be a bigger bore

[MCRider]

No, I think it's correct, from memory, modern MC pistons are smaller in dia, but have longer strokes. I didn't worry about all that crap, I just bought a modern MC and it works great. Cheers, Terry.

Yeah, like many things in life its easy to over think some things.

Early on, when i converted, I just went to a salvage yard (pre-eBay) and got a MC from a dual brake kawasaki. Whatever it was, it worked great.

On current project I got one (eBay) from a VF500F. Works great.

No math.

[Viktor.J]

of course the "first" feeling is that a MC from a "modern" bike would work just fine. But then you read these threads and quotes about this and that...

And since im still a student i dont want to buy a MC for 100$ good for nothing (that easy locks the brakes or no difference at all)..

But of course whats relevant isnt really the area of either the caliper piston or the MC cylinder... Its the Volyme that mathers = the pressure rise...

[bjbuchanan]

The chart is calling for the smaller piston it if taken simply as a factor it will increase pressure in the line. Other factors like volume matter but thats the idea the chart is basing it off of.

[Viktor.J]

maybe im failing on the english here... but what do you mean ?

"other factors like volume mathers but thats the idea the chart is basing it off" ?

The volume is directly linked to area... so its not a "other factor"...

If something mathers its the volume, not area. the area direct affects the volume...

[70CB750]

I run dual disc on my CB750 with the original MC and it works well. You dont have to replace the MC if you dont want to.

[MCRider]

I run dual disc on my CB750 with the original MC and it works well. You dont have to replace the MC if you dont want to.

Actually, I did that also. I don't remember any issues. Then it went bad (return hole plugged, but I didn't know at the time how to fix it) That's when I went to the salvage yard, and got the Kaw duallie MC.

So my rec would be the saame, use the stocker first.

[Don R]

I run dual disc on my CB750 with the original MC and it works well. You dont have to replace the MC if you dont want to.

Mine works well too, it takes more lever travel to move both pistons but I like mine. I can squeal the front tire in a practice emergency stop.

[Viktor.J]

OK, thanks for all the input guys !

I think this is what i should do, Install the other disc, refresh the pistons, change braking pads and change the braking lines to braded. And try that out and then maybe change the MC..

I know its been talking about the GL1000 MC, but they are (at least in Sweden) expensive as hell.
David silver parts has a "pattern" part of the honda cb750f2 MC, for a reasonable price.

If any one other has more MC tips, its welcomed