Twin disc conversation master cylinder bore?

[SpannerBrat]

Hi there,
I seem to have managed to fettle an extra (rhs) calliper into life & fit it to the 750SS forks I got with the “box of rusty bits” bike I originally bought. All “looks” decent (i’v not had anything working yet) but I need to buy new levers & so I wanted to find out from you guys what size of master cylinder I need to hunt for ont’interwobble? Searches are throwing up 5/8” as a likely candidate. If any of you have any cool looking alternative suggestions for levers, i’m all ears.
Also, have any of you advice on the best twin disc hose arrangement? I  prefer the look of looping a link hose from one side to the other but suspect that this could a) not be as powerful as running a full length hose to each side & b) could also be more problematic when it comes to bleeding (but both of those are guesses).
As ever, thanks for any input. This forum has been a great help in keeping my build going (& importantly, going in the right direction).

[Bryanj]

GL 1000 master cylinder and if you get the one with the brake light switch fit the second hose where the hydraulic switch is, if not a double length banjo bolt

[SpannerBrat]

Thanks Bryan,
The bore of the GL MS seems to be 14mm but 16mm is the number being thrown up by searches for 550/750 MSs. Could you educate me on which I should be tending towards & why (i’m assuming from your recomendation of GL, the smaller bore is better)?

[Bryanj]

I twindisced a 500 back in 76 with one caliper in front and one behind as no 550 front ends were available and i used a GL 1000 master as i could get a Lucas pattern one ( cosmetic second from jal supplier) It worked perfectly but be carefull of big handfulls on very wet roads as you fall off! Got the tee shirt and video for that one

[petermigreen]

be carefull of big handfulls on very wet roads as you fall off! Got the tee shirt and video for that one

Any chance of posting the video?

[cb750stu]

Hi I've got a twin disc set up my 76 750 F1 SS and I'm just using a standard master cylinder works really well

Sent from my Pixel XL using Tapatalk

[SpannerBrat]

Thanks for the comments.
I’d be ok using anything that works well tbh (GL, CB, whatever). I’m just a bit confused on the bore size situation. Is there a simple rule of thumb like “the larger the bore, the less grip force required (or visa Verda)?
There’s so much choice out there from cheep (£20 & even less) units that have surely got to be made of cheese, to ridiculous (£hundreds) & from onboard reservoirs to “remote” styles. The bike’s a mixture of all sorts of parts & is planned as a brat/cafe so anything goes really. I’m just trying to understand what exactly I’m looking for to give me decent performance. I’ll not be blasting around the N.Yorks moors wearing out knee sliders at every opportunity but equally, i’d like to know i’ve got some effective anchors if/when needed.

[cb750stu]

Thanks for the comments.
I’d be ok using anything that works well tbh (GL, CB, whatever). I’m just a bit confused on the bore size situation. Is there a simple rule of thumb like “the larger the bore, the less grip force required (or visa Verda)?
There’s so much choice out there from cheep (£20 & even less) units that have surely got to be made of cheese, to ridiculous (£hundreds) & from onboard reservoirs to “remote” styles. The bike’s a mixture of all sorts of parts & is planned as a brat/cafe so anything goes really. I’m just trying to understand what exactly I’m looking for to give me decent performance. I’ll not be blasting around the N.Yorks moors wearing out knee sliders at every opportunity but equally, i’d like to know i’ve got some effective anchors if/when needed.

This is what I use , one brake line coming from M/S then into splitter to the two calipers , works really well plenty of braking power

Regards

https://www.davidsilverspares.co.uk/CB750F1-(Single-overhead-cam)/part_123909/

Sent from my Pixel XL using Tapatalk

[Bryanj]

Peter, you need to be fluent in heiroglyphics to watch that video mate!

[Bryanj]

I worked on several twindisced Hondas in the dim past and whilst the standard cylinder will work we always found them a swine to bleed completely or you got a lot of lever movement. I did a lost of piston/seal reasurch at the time and found the GL had the same piston bore on the caliper as the 500/750 (but differant shaped pistons) and working on the priciple that Mr Honda had done the maths for me I used that master, also Girling had just got into that market and Wassells sold me a master super cheap.

[matthewmosse]

On a cafe racer you might look at old circa 78 bmw master, that ran a cable from the bars to a remote master under the tank, made for a cleaner appearance on the bars but slightly more bits to maintain or go wrong, on mine I ballooned the original brake lines and snapped the cable within a week of buying the bike, basically I thought it looked ropey and tried pulling as hard as I could in the car park, just to see if my feelings were right about the incompetence of the mot tester who gave it a ticket, glad I did that in a car park at the standstill. Don't trust mot man to catch every fault, on that he wouldn't reasonably have spotted the cable bein internally scrap, but should have failed the frail looking rubber brake hose I would have said. Very perished.

[kevski]

I used a gold wing one when i twinned up my 500k3 in the late nineties.

[Colonial-Clive (yindi)]

I have twin discs on mine I used original master cylinder worked but not very well bought fancy race bike one worse than original then got gl from DSs and now stops on a sixpence!
PS I also updated to braided hoses.

[Johnwebley]

I used a gold wing one when i twinned up my 500k3 in the late nineties.

  same for me,and only £30 from David Silver

[SpannerBrat]

Hi all,
Thanks for the comments & suggestions so far. I thought to try the std cylinder/lever arrangement that I have here but on offering it up to the bike, it turns out that the cylinder exiting from the brake unit fouls against the clip-ons i’m using. So i’m expecting the GL unit wouldn’t work either & therefore  it seems a plan C might be needed. That being the case, the BMW option (thx Matthew) is worth investigating but I find myself wondering what a radial setup would work like (with remote “sample pot” - fortunately i’m not completely adverse to the look). Has anyone any guidance on what I should be looking for there (appropriate model, year, part generally from a larger capacity bike or does it really matter)?