75 model front brake master cylinder option.

AlanW

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I’ve read some of the threads here suggesting changing to a smaller diameter brake master cylinder to improve braking on my 1975 B. Planning on replacing the brake lines to braided so wanted advice please on options for the MC from you learned chaps. I’ve looked at this as a possibility and it’s on offer at the moment - Brembo 10462082. 11mm. As ever ,look forward to hearing your valued opinions. 🎄
 

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One of my complaints about my XS650 was the wooden feel to the front brake. Hardly any lever movement and no feel. Replaced the master cylinder - bought a Yamaha scrambler one on ebygum, undoubtedly Chinese, nominally 1/2" piston, and that change transformed the brake. Enough feel to use the brake hard. The 11mm piston should give even more feel.
 
I’ve read some of the threads here suggesting changing to a smaller diameter brake master cylinder to improve braking on my 1975 B. Planning on replacing the brake lines to braided so wanted advice please on options for the MC from you learned chaps. I’ve looked at this as a possibility and it’s on offer at the moment - Brembo 10462082. 11mm. As ever ,look forward to hearing your valued opinions. 🎄
A good option I would say. Two things. The banjo thread on Brembo is 10 x 1mm rather than 10 x 1.25 on other makes. And the mirror thread won't be left hand to take the Yamaha mirror stem. You can buy a left hand thread part from Brembo, I bought one a while ago.
 
Thanks guys I’m pretty sure I’m going for it.
- Steve, it’s a standard twin piston caliper. The MC on the bike at this time has a right hand thread mirror.
- Raymond, current braking is definitely wooden, plus you need a King Kong grip to get it stopped. Forward planning definitely required. 😳
 
I think your original MC is 5/8", or about 16mm? I've read that the 14mm MC from a later model works well. I put an 11mm on my later model and it works well, but that's only a 3mm reduction from stock. For you, it would be about 5mm, and that might be too much. The only "issue" I encountered with the 11mm MC was increased lever travel. I've grown used to it but on yours, the lever may pull in to the grip. I tried a 1/2" MC (12.7mm) on my other 650 and while it's a slight improvement, it doesn't give as much "feel" as the 11mm one does. However, it doesn't increase lever travel much either. It might be just right for your needs.
 
Thanks 5twins. To the uninitiated can you please advise why the smaller bore results in increased lever travel?
Brakes are hydraulics; You put movement pressure on one cylinder to create movement pressure in another
to change the amount of pressure exerted by the caliper you either make the wheel cylinder bigger (modern multi piston calipers) or make the master cylinder smaller. (the easy way out on our old bikes). The downside is the smaller master cylinder moves less volume so it's best to have teflon/stainless brake lines that don't expand (balloon) like old rubber lines do.
https://blog.bikernet.com/guide-to-hydraulic-ratios-and-choosing-master-cylinders/
mastercylinder ratio chart.gif
 
Thanks again 5twins. As ever, I really appreciate the advice from you guys. I’m going to replace the (almost) 50 year old original brake lines first. Then make a decision on Master Cylinder based on the helpful replies to my original post. Brembo also do a 13mm version 🤔
 
Honestly, I'm not sure why, all I know is it happens, lol. When I first installed my 11mm, the lever pulled back to the grip. Well, that was no good, but from all I'd read, this size was supposed to work well on my bike. I thought the MC may need an overhaul, so I bought a kit (and it wasn't cheap, lol). But before installing it, I took a close look at my lever. Turns out it was bent, not up and down but the curve in the handle was bent straight. So, I bent a curve back into it and ..... problem solved, the lever no longer pulls all the way back to the grip.
 
Thanks 5twins. To the uninitiated can you please advise why the smaller bore results in increased lever travel?
Well, it's simple physics. The brake pistons have an area in sq mm - if piston diameter is 42.8 mm then area is approx 1830 sq mm. We need to push the piston a small distance to grip the disc, say 1mm? So we need to move approx 1830 cu mm of fluid.

A 14 mm master piston has area approx 615 sq mm and needs to move about 3mm to push that much fluid but an 11mm master piston, area approx 380 sq mm, needs to move nearly 5mm to push the same amount of fluid. More travel equals lighter pull on the lever and more feel, as long as you don't have the lever coming all the way back to the bar!

It's been suggested the large master piston was optimised for a twin-disc set-up?

The above assumes that brake fluid is not compressible. Spongy brakes means they should be bled of air and/or fluid replaced.
 
I've heard it said that early disc brakes were purposely made less than great to make them feel similar to drum brakes. Though this may well be an urban myth. I've had bikes with really great drum brakes.

If you are looking for a one piece brake line I can recommend Hel. You can build a custom line on their website. It cost me around £30.
 
I've heard it said that early disc brakes were purposely made less than great to make them feel similar to drum brakes. Though this may well be an urban myth. I've had bikes with really great drum brakes.

If you are looking for a one piece brake line I can recommend Hel. You can build a custom line on their website. It cost me around £30.
Early disc brakes were pretty rubbish anyway, first disc I had a was a 1977 CB550 and it was hopeless in the wet. Japanese stainless steel discs usually were - probably not meant to go out on rainy days? Italians fitted cast iron discs which worked in the wet but rusted badly and seized up when left in the garage. Must have been as heavy as a different sort of anchors. Then came sintered pads and at last we had good brakes.

I'm currently running a drilled EBC disc and their pads - together with a one-piece braided Hel line and the smaller-bore master cylinder mentioned in post #2 above. This gives me all the stopping I want. At the risk of annoying regular readers, I'll mention yet again the value of regularly practicing hard braking - approaching a speed limit, after checking the road behind is clear, I'll give the lever a progressive squeeze and it's quite surprising how much retardation you get before the tyre starts to chirp . . .
 
So…. Definitive answers please based on previous posts.. will the Brembo 11mm MC be okay? The current set up bites straight away so a little more travel on the lever (which is not bent @5 twins😀) should be fine - I hope.
 
Definitive answer? Yes, the Brembo 11m will be fine. Would recommend changing the line for single-piece braided at the same time.

There will be cheaper m/c options but going by reputation the Brembo will be good quality kit. If you still have performance anxieties (ooh err) then further upgrades are available.
 
Thanks Raymond! That’s it - new 11mm MC and new braided hose from HEL performance.
Appreciate, as ever, all the helpful feedback from everyone.
 
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