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11 / 12mm front brake master cylinder?

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As I mentioned elsewhere the front brake on the little Honda is very wooden feeling with barely any travel. The standard master cylinder is 5/8" diameter - or so it says on it....

I had a similar problem with the Guzzi V50 and fitted a master cylinder from a Honda model but have mislaid the information I used at the time. I'm looking for one either 11mm (preferred) or 12mm bore with a 10mm mirror mount. Does anyone know where I can find out what might be a suitable replacement?

Hi Ian

I just measured the spare master cylinder from my old CBF500 and that is 15mm which I know is too big for your needs, but it wasn't that bad in use on my GN250. It certainly made that brake feel less wooden.


Thanks for that - it does depend on the caliper piston diameter as well. I'm trawling through ebay listings for repair kits which usually give the diameter so I can refer back to the actual master cylinders. It seems CBR250RR and CBR125RR master cylinders are 11mm and there are quite a few available as well as cheap aftermarket ones which of course are an unknown quantity.

I'm also not entirely sure I've got the existing bore correct as it says 5/8 on the casting which I make 16mm so 11mm would be too small. 12.5mm or 1/2" would be better. Of course what is fitted may not be what it left the factory with.

I've got a 1/2" diameter master cylinder which is from a CBR500. I am however slightly concerned - the lever action is very much improved but travel is much greater. I did expect this but it's more than I thought - explained in part by the fact that the lever pivot to actuating point distance is substantially less than on the original (around 20mm as opposed to 25mm or so) so multiplying the mechanical / hydraulic advantage. It down't quite come back to the twistgrip but not far off.

There may still be a small amount of air in the system although I've bled it carefully so will leave to rest for a while.

After much toing and froing have ended up with an ebay Chinese special This has a 14mm bore but 20mm pivot to abutment distance. For various reasons have only just managed to try it. The feel is much better than the original but not as soft as the last one I tried.

A brisk 20 miles showed me that the front brake is much improved both in feel and effectiveness but still needs a substantial handful to get a decent effect. All in all the mod seems to have been worthwhile and I enjoyed the ride very much.

Just one point about the ebay item - although it was listed as being located in the UK it actually took around 10 days to arrive and clearly came from China, so not one to use if you're in a hurry.


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