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CB750K Master Cylinder Bore

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K2-K6:
Yes agree, the deburr would need empty system and cleanup to achieve and marginal gain to consider if you're intent on chasing to that level.

Interested to hear if you feel any worthwhile change from heating it.

For interest.
The deburr and things like that seek to keep more fluid in the line as the lever is retracted and so set the caliper piston slightly closer to the disc in it's travel caused by the piston seal. Ultimately this tries to keep the free lever movement before the mastercylinder piston seal closes the port to a minimum, then holding the lever's mechanical advantage higher in it's range to optimise that geometry and help lever feel etc. This is the portion of lever interaction with the piston face as it's more direct/effective at certain angles rather than wiping across the face of that piston and causing friction that's unwanted. These aspect all contribute to that wooden feeling often described as friction builds up when you need higher operating pressures.
You can see on some current mastercylinder designs that they feel with this by using paired roller bearings for lever pivot (with using moly grease on old plain type levers a good idea) to help improve feel, some with variable ratio lever geometry too.

Certainly illuminating when you try a newer system as they aren't that much different to older type but with careful attention paid  to ratio and friction bringing real advantages.  These old disc are easily large enough to make significant braking torque, that's if you can squeeze them hard enough to get there.

Lobo:
Interesting. But if I read this right the lever will begin to work sooner… in layman’s terms?
Holding the lever’s mechanical advantage higher in its range is counter active to my stretched out fingers and their mechanical disadvantage!

I’m not keen to de-standardise my ‘750, but a shortening of the lever’s piston lug may satisfy my need; ie closer to the bars lever position before braking action, giving my fingers more comfortable advantage.

Wrt heating the disc suggestion, not getting as much riding as I want in this hemisphere…. seemed to have swapped weather with the UK … far too much rain of late & the retirement plan failing dismally. You stole our sunshine the past few months 😂.

Thanks Nigel… always appreciate the technical learnings…

Trigger:
If you require more travel on the brake lever, grind the alloy down a bit on the part that pushes the M/C piston. A little at a time until you are happy with the travel.  ;)

Lobo:
Thanks Trig, that indeed was what I’ve done.

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