CB750K Master Cylinder Bore

[Lobo]

Can anybody verify the standard piston diameter here please… my manuals suggest 14mm, but within the reservoir suggests 12mm. Furthermore, prior to any knowledge on this topic I simply ordered a Yamiya service kit and fitted it… without any dramas. Sadly I’ve binned the old piston and so can’t easily measure / verify the bore now.

I ask because the front brake has a woody feel to it; at abouts 20 mm (lever ) ball travel it’s biting, and within a very short further pull I’m at max effort.

I’m pretty sure the reservoir is original, and as stated, has just had new internals. The lines are OEM, ie NOT the braided type. I’m using sintered pads and the stopping performance is fine. To clarify, I’d just like a bit more travel / feel within the system.

[Bryanj]

That was a common complaint of testers back in the 70's. If the lever is solid and the brake works plus all the pivots free and greased, thats what it is

[Lobo]

Thanks Bryan; good to know. Yep, EVERYTHING about the brake system is tickety-boo so I guess I’m stuck with it.  Cheers.

[K2-K6]

As noted above, that does sound like everything is in order for one of these brake system.

Honda were very, very conservative with these early production disc brakes (you can understand why, along with them being almost impeccably reliable from a component point of view) but do have as a consequence a fairly dull response to lever pressure.

Bore I've got from manual as 14mm and same as you Simon. Don't think the 12 in the casting refers to piston bore (could be wrong) you'd need to pull the piston out to really understand but looks all ok as is really. This gives a hydraulic ratio of 7.44 advantage to the lever which is pretty low by comparison to new system.

I feel that the sintered and ceramic pad materials need slightly higher line pressure to feel good, and sometimes experience will give opinion as you find that they feel wooden. Certainly don't have a very quick "grab" on application is routine comment, especially when compared to current systems in use.

I believe they can benefit from proscribed running to optimise  bedding in though to make best use of a well setup system. 50 miles of general use should get the pads something like flat/topographically mated to the disc in use, followed by a higher range heat cycle to get both disc and pads working well. That would entail riding with brake reasonably applied (run in third gear to give load) to hold pace down for about a mile to get the disc properly hot and enough to easily sizzle water if contacted. If hot enough at this point, don't stop the bike but just ride to let it cool brake to lower temp now.
Usually you'll feel the bite changing as you put more heat into the brake while running it, that's a good signal it's getting where you want it to be. This is especially true if the disc has been cleaned with brake cleaner or left around in a workshop for a while. Don't clean the disc again afterwards else you'll need to do this again.

Something else that can be worthwhile, the little hole in mastercylinder reservoir that feeds the piston/bore with fluid, deburr that entry point possibly with a hand used decent multi blade counter sync to reduce obstruction there. That's a production foible and shouldn't ideally be there.  It affects the system as you release the brake pressure with any unwanted resistance at that point causing the caliper piston to be withdrawn further that optimum. It's a small detail but can offer improvement in marginal feel and brake system response. Ideally would be a miniature bell mouth that's polished to improve entrance flow.

[Lobo]

…thanks Nigel; will try the higher range heat cycle, but put the deburr on hold as it sounds like a thorough flush subsequently needed, even assuming metallic waste bits don't damage the (all new) seals.

[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.